DISEASES OF FIELD PEA (Pisum sativum L.)
IN THE PEACE RIVER REGION OF ALBERTA
by Paul Laflamme PAg.
B.Sc. Agriculture - University of Alberta - 1986
THESIS SUBMITTED IN PARTPAL FULFILEMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF PEST MANAGENEENT
in the Department
of
Biological Sciences
O Pau1 Laflamme 1998 SIMON FRASER UNIIVERSITY
July 1998
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Abstract A survey of diseases of field pea was done in the Peace River Region of Alberta during June to August of 1997. A total of 103 fields were surveyed duruig the seedling stage, at f l o w e ~ g and after pod set, and over 15,000 samples were obtained The incidence of root rot was determined by recording the number of plants with discolouration on the roots. Root rot severity was determined by rating the area of each root showing signs of discolouration, on a scale of O - 4. Root rot and foliar disease-associated fungi were identified by plating symptomatic tissue onto potato dextrose agar. Mean incidence and severity of root rot for the three sampling times was 88% and 1.2,88% and 1.5 and 95% and 2.3, respectively. Fusariurn spp. were the predominant fungi isolated fkom root tissues. Ascochyta spp. and Sclerotinia sclerotiorum were the fun@ isolated most fiequently from foliar tissues. Symptorns of mycosphaerella blight were found in ail fields by the third sampling time and symptoms of sclerotinia rot were observed in 54 fields by the third sampling time. Significant differences in root rot incidence and severity were recorded between different areas of the Peace with the Falhermeace River area having the lowest overall level of both root rot incidence and seventy. Growers were also surveyed on their agronomic practices. E s information was statistically analyzed for relationships between agronomic practices and root rot incidence and severity. When averaged across the 3 sampling times, the analysis revealed that many agronomic practices reported in this survey were correlated with small but significant diffaences in root rot incidence but only a few with significant differences in root rot severity. Fields that had been planted to canola in 1996 had significantly higher root rot incidence than other fields. Fields planted to legumes in 1995 had significantly higher root rot incidence and severity than other fields. Growers who used certified seed had a significantly higher root rot incidence and severity than growers who used common seed. Pea seed treated with metalaxyl had the highest root rot incidence while pea seed treated with thiram had the lowest. Use of peat based seed inoculant resulted in the lowest root rot incidence and no inoculant resulted in the highest. Use of fertilizer resulted in higher root rot incidence and severity than when no fertilizer was used. Banding of fertilizer resulted in the highest root rot incidence and severity while applying fertilirer with the seed resulted in the lowest. Minimum tillage resulted in the highest level of root rot incidence while zero tillage resulted in the lowest. Pea fields that were rolled immediately &er seeding had the hîghest root rot incidence while fields that were rolled before emergence or were not roIled had the lowest. Growers who sprayed their pea fields with herbicides had significantly higher root rot incidence and severity than growers who didn't spray Disease in previous pea crops and herbicide residues had no significant effect on root rot incidence. A disease management program for field peas grown in the Peace River region of Alberta is presented.
Acknowledgements
1 want to especiaily thank my wife Diana, for all her support, encouragement and patience while 1 was working on this project. Also my three children, Sara, Chantal and Jeremy for leaving their fnends and taking a year out of their Lives to move to Bumaby so that 1 could do the course work for this Masters degree. I hope it was as much of an adventure for them as it was for me. I also a-ant to thank Dr. James Rahe and Dr. Zamir Punja for al1 their help and advice in the editing of this thesis. A big thank-you also goes to my employer, Alberta Agriculture, Food & Rural Development who provided me with the opportunity, support and financial resources to undertake and complete this Masters degree. And hal ly , thanks to the Alberta Agriculture Research Institute and the Alberta Pulse Growers Commission for providuig funding for this project.
Table of Contents
Approval ............................. .. .......................................... ..................................................................... Abstract .......................................... .,.,
Ac know ledgements ... .. .. ... ........................................................................................ Table of Contents ........................................................................................................ List of Tables ............................................................................................................ List of Figures ........................................................................................................ Chapter 1 -Background ...............................................................................................
......................................................................... 1.1 History of Pea Production
1.2 History of Pea Production in Canada ....................................................... ................................................................................. 1.3 Climate of the Peace
................................................................................................ 1.4 Objectives . . Chapter 2 - Pea Disease Descriptions .........................................................................
................ 2.1 Soil-borne Diseases . Seedling Blight, Seed Rot and Root Rot
2.1 1 Fusarium Root Rot ......................,............................................ 2.12 Fusarium Wilt ............................................................................. 2.1 3 Pythium Root Rot ........................... .... ... .. .... 2.14 Rhizoctonia Root Rot .................................... .... .......................... 2.1 5 Aphanomyces Root Rot .............................................................. 2.16 Thielaviopsis Root Rot ............................................................... 2.17 Prevention & Control of Seedling Blight, Seed and Root Rot ....
2.2 Foliar Diseases ......................................................................................... 2.2 1 Ascochyta/Mycosphaerella Blight ..............................................
. . 2.22 Sclerotmia Rot ............................................................................. 2.23 Powdery Mildew ......................................................................... 2.24 Downy Mildew ...........................................................................
.................................................... 2.25 Gray MoId .......................... .. .................................................... ......................... 2.26 Anthracnose .,,
Page
Table of Contents
Page
2.2 Foliar Diseases (cont.)
.................................... 2.27 Alternaria Blight- .......................... .... 31
2.28 Septoria Blotch ............................................................................ 32
................................................................... 2.29 Cladosponum Blight 33
2.210BlackLea.f ........................................................................... 34
2.3 Bacteriai Diseases ..................................................-.........................-..... 35
2.3 1 Bacterial Blight .............................. ... ......................................... 35
2.32 Brown Spot ................... .... .................................................... 37
................ ...........................................................-- 2.33PinkSeed. .. 38
2.4 Vinises ...................................................................................................... 39
2.4 1 Pea Seed-borne Mosaic Virus .................................................... 40
2.42 Pea Enation Mosaic Virus ........................................................... 41
...................................... .............. 2.43 Bean (Pea) Leaf Roll Virus ... 42
2.44 Pea S treak Virus ................................................................... 43
2.45 Pea Stunt ................................................................................. 44
Chapter 3 - Field Survey for Pea Diseases .................................................................. 46
3.1 Materiais and Methods .............................................................................. 46
............................................. 3.1 1 Selection of Fields to be Sumeyed 46
3.12 Sampling Metho d. ....................................................................... 48
................................................ 3.13 Root Rot Pathogen Identification 52
3.14 Foliar Pathogen Identification ................................................ 52
3.15 Statistical Analysis of Survey Questionnaire ............................... 53
3.2 Results ........................................................................................................ 54
3.21 Incidence and Severity of Root Rot .......................................... 54
vii
Table of Contents
Page
3.2 Results (cont.)
3.22 Pathogen Identification ...................... ... .................................... 54
3.22 1 Root Rot Pathogens ....................................................... 55
3.222 F o l k Pathogens ............................................................ 55
3.23 Survey Questionnaire Results .................................................. 56
................ 3.24 Resuits of Statistical Analysis of S w e y Questionnaire
3 -24 1 Differences Between Areas .......................................... 3.242 Cropping History ................... .. ............................... 3.243 Disease in Previous Pea Crops .......................... ......... .... 3.244 Seed Source ................................................................... 3.245 Seed Treatment ............................................................. 3.246 Inoculant Formulation ................................................... 3.247 Fertilizer Use ................. ... ....................................... 3.248 Fertilizer Application Method ....................................... 3 -249 Tillage. ..........................................................................
........................................................................... 3.250 Rolling
3 -25 1 Herbicide Use ................................................................ 3 . 252 Herbicide Residues ........................... ... .........................
3.3 Discussion ................................................................................................. 3.3 1 Effect of Agronomie Practices on Root Rot Incidence & Severity
3.3 1 1 Differences Between Areas ........................................... 3 -3 12 Cropping Histo~y ........... :: ............................................
................................................................... 3.3 13 Seed Source
3 -3 14 Seed Treatment ............................................................. 3.3 15 Inoculant Formulation ...................................................
viii
Table of Contents
Page
3.3 1 Effect of Agronomie Practices on Root Rot Incidence & Seventy (cont.)
3.3 16 Fertilizer Use ......................... .... ............................ 81
....................................... 3.3 17 Fertilizer Application Method 82
3.3 18 Tillage ........................................................................... 82
3.3 19 Rolling ..................................................................... 83
3 -320 Herbicide Use .............................. ... ...................... 83
Chapter 4 - Pea Disease Management Program ................ .... ............................... 85
4-1 Cultural Practices ...................................................................................... 85
4.11 Conventional Tillage .......................... .. .................................... 86
4.12 Conservation Tillage ................ ... ............................................ 87
4.13 Other Cultural Practices ............................... ... ............................ 90
4.2 Host Resistance ........................... .. ............................................................ 91
4.3 Chemical ControIs .................................................................................... 94
4.4 Proposed Pea Disease Management Program ......................................... 96
Chapter 5 - Conclusions and Recommendations ........................................................ 101
................................................................................................................. Appendix 1 104
........................................................................................................... Literature Cited 106
List of Tables
Page
Table 1: Incidence and severity of root rot in the Peace River region in 1997 ......... 54
Table 2: Mean acreage, standard deviation and range of fields surveyed. ..............Y.. 56
Table 3: Cropping history of fields surveyed ......................... .. .............................. 58
Table 4.1 : Number, percent and Chi-square probabilities
for sources of variation by overall root rot incidence .................. ... ..... 71
Table 4.2: Number, percent and Chi-square probabilities
for sources of variation by overall root rot incidence ................................. 72
Table 5: Overall mean severiiy, coefficient of variance, F-value
and Pr > F for root rot sevenv by source of variance ................................. 73
List of Figures
Page
Figure 1 : Acreage of Field Pea in Alberta 1982-1997 ........................................... 4
Figure 2: Acreage of Field Pea Ui Peace Region of Alberta 1986- 1997 ........... .... 4
Figure 3 : Map of Survey Area and Location of Fields Surveyed .............................. 47
Figure 4: Field sampling method used to obtain pea plants for this study .................. 48
Figure 5: 1997 Growing Season Precipitation - Grande Prairie ................................. 50
............... ............ Figure 6: 1997 Growing Season Precipiîation . Falher ........ 50
..................................... Figure 7: 1997 Growing Season Precipitation . Peace River 50
.................................... Figure 8: 1997 Growing Season Precipitation . High Level 50
...................... Figure 9: 1997 Growing Season Precipitation . Overall Peace Region 50
Figure 10: 1997 Growing Season Mean Temperature - Grande Prairie ..................... 51
Figure 1 1 : 1 997 Growing Season Mean Temperature - Faber ................ .... .......... 51
Figure 12: 1997 Growing Season Mean Temperature - Peace River .......................... 51
Figure 13: 1997 Growing Season Mean Tempehue - High Level ........................... 51
Figure 14: 1997 Growing Season Mean Temperature - Overall Peace Region .......... 51
.................................. Figure 15:RootRotIncidenceinArea5 -Faher/PeaceRiver 78
Figure 1 5: Root Rot Severity in Area 5 - Falherff eace RWer ..................................... 78
Chapter 1 - Background
1.1 History of Pea Production
Peas (Pisurn sativum L.) have been cdtivated by humans for close to 10,000
years, almost as long as wheat (Trificurn aestiwm L.) and barley (Hordeum vulgaare L.).
Peas provide rnuch-needed protein to complement the starches of the cereals. The
appearance of a smooth seed coat is thought to be the most reliable indicator of
domestication. Wild peas generally have a rough or granular seed surface, while
domesticated peas are characterized by a smooth seed coat. Samples of carbonîsed
smooth coated pea seed have been found during archeologicai excavations in early
Neolithic villages of the Near East dating fiom 7000 to 6000 B.C. Peas made their first
appearance in Europe in the Neolithic period in various agricultural settlements such as
in Greece (5500 B.C.), in Bulgaria (4330 B.C.) and in Romania, Czechoslovakia, Austria,
Switzerland and the Iowa Rhine Valley (4400 - 4200 B.C.). Earliest finds of peas in
western Europe corne from the Bronze Age (Zohary & Hopf 1973).
Peas were originally calledpisos by the Greeks andpisum by the Romans. When
they first showed up in England they were called "peason", then 'peaseYy or "peasse" and
this was eventually shortened to "pea", the name in use today ( M m 1977).
Domesticated peas, Pisum satiwm L., have been traced back genetically to the
wild pea type P. humile, which is still found growing in the oak park forests of the Near
East (Zohary & Hopf 1973; Cousin 1997). Peas probably originated in Abyssinia and
Afghanistan with areas in the Meditemean region col onized later (Cousin 1 997).
Vavilov (1 95 1) in his search for the origin of plant species listed two "centres of
diversity" for peas. These were the centrai Asiatic centre and for larger seeded varieties.
the Mediterranean centre.
1.2 Ristory of Pea Production in Canada
Peas were first grown in Canada by French settlers who used them for making
"pea soup", and they were popular in the diets of pioneers who helped to settle the west.
Dry peas have been grown on a lunited acreage in western Canada since it was Grst
settled more than 100 years ago (Slinkard et al. 1994b).
Peas were a popular crop in Eastern Canada in the late 1800's. An average of
29 1,384 hectares were grown each year fiom 1883 - 1902. Production declined however
as other crops became more common. By 1970, only 25,09 1 hectares of peas were grown
in Canada with Manitoba accounting for 70 percent of this area (Anonpous 1997a).
Peas did not become popular again until the wheat glut of the mid 1970's. The demand
for field peas was increasing and western Canadian f m e r s were willing to diversi@
their operations to fil1 the need (Slinkard et al. 1994b). Western Canadian f m e r s
recognized that field peas had mmy desirable agronomie characteristics they were
looking for. They could be seeded and harvested with conventional farm machinery, they
fixed their own nitrogen and they fit well into crop rotations heIping to break many
disease cycles.
Alberta f m e r s embraced this new Cinderella crop. The fxst record of field peas
in Alberta is in 1 890, when 43 hectares were seeded (Anonymous 1 996a). By 19 10, there
were 12 1 hectares grom (Woychuk et al. 1972). Production fluctuated between 162 and
1 1,736 hectares until 1987 when a record 26,305 hectares were grown (Anonymous
1996a). Interest in field peas was sparked by bigh pBces for green peas because of a
drought in the Pacific Northwest of the USA (Slinkard et al. 1994b). Alberta had been
growing mainly green peas and reaped much of the benefit fkom this pnce increase.
Alberta has the highest pea yields in Canada, averaging 35 percent higher than either
Saskatchewan or Manitoba. In 1997, Saskatchewan had the highest acreage (607,000
hectares) folowed by Alberta (155,800 hectares) and then Manitoba (83,000 hectares)
(Anonymous 1997b).
Figures 1 and 2 illustrate how production has increased in Alberta since 1982 and
in the Peace River region since 1986. Reasons for the dramatic increase in acreage of
field peas are nurnerous. One of the main reasons is that markets for animai feed and
human consumption have increased both domestically and intemationally and that
growing field peas is now very profitable (Anonymous 1996a). The introduction of new
semi-leafless, erect and high yielding pea varieties has also helped to keep f m e r s
interested by making harvesting easier and increasing returns.
Year
Figure 1 : Acreage of Field Pea in Aiberta 1982 - 1997
1986 1990 1991 1 QB2 1993 1884 1995 1996 1897 Yaar
Figure 2: Acreage of Field Pea in the Peace River Region of Alberta 1986 - 1997
Source: Alberta Agriculture, Food & Rural Development - Statistics & Production Economics Branch 1998.
5
More than 70 percent of the peas grown in Canada are exported. The largest
buyers of Alberta peas are Spain (52,332 tonnes), Belgium (5 1,2 1 1 tomes), Cuba
(25,220 tonnes), India (22,682 tonnes), France (1 1,922 tonnes) and the Peoples Republic
of China (1 l,82 1 tonnes) (Pekalski 1997). Within Canada, the pork industry in Alberta
and Manitoba accounts for the largest domestic connimption of peas (Anonymous
lW6a).
1.3 Climate of the Peace River Region
The climate of the Peace River region is characterized by relatively cold winters
and moderately warm summers (Reeder & Odynsky 1965). Though annual precipitation
is low, it is relatively well distributed during the growing season. Moderate temperatures
during the growing seson also help to keep evapotranspiration low. Long s u m e r days
prornote rapid development of crops and rnost mature within the fiost fiee period (Carder
1965). The region can be divided into two distinct climatic areas; the Upper Peace and
the Lower Peace (Carder 1965; Carder & Siemens 1971). The Upper Peace (55 O to 57"
latitude) encompasses the bulk of the agricuftural area in the region. The lower Peace
(58 O to 60" latitude), so named because of the lower elevation is slightly warmer in
summer but much colder in winter. The Lower Peace is also generally drier but has less
wind so evapotranspiration is reduced. Longer daylength in the Lower Peace also
promotes more rapid growth of crops and compensates for the shorter growing season
(Carder & Siemens 1971). In general, July is the w m e s t month of the year followed by
August and then June (h4cKenzie & Hall 1976). The 30 year (1 967- 1997) average
growing season (May to September) precipitation ranges from 306.7 mm in Grande
Prairie, to 242.2 mm in Peace River, to 257.8 mm in High Level. The 30 year average
annual growing degree days greater than 5°C for the region is 1287.9 (Anonymous
1998~). Frost fiee periods (0°C) vaiy fkom 113 days at Grande Prairie in the Upper Peace
to 9 1 days at Fort Vemülion in the Lower Peace. (Anonymous 198 1). The agicultural
potential of the region is improved substantially by the long summer day length. It should
be noted that large variations in temperature and precipitation exist fkom area to area
within the region and fiom year to year (Carder 1965; Carder & Siemens 197 1;
McKenzie & Hall 1976).
Because field peas have been grown on a large scale for only a short time in the
Peace region, very little work has been done on pea diseases. Hamison & Laflamme
(1996) did a field pea root rot survey of 38 fields in the Peace region in 1995. They f o n d
that root rot was present in al1 fields surveyed. The main disease organisrns identified
were Fusarium species. Uther diseases observed were sclerotinia rot (Sclerotinia
sclerotiomm (Lib. de Bary), ascochyta blight (Ascochyta spp.), downy mildew
(Peronospora viciae (BRk.) de Bq), powdery mildew (Evsiphe pisi Syd.) and gray
moId (Bavtis cinerea Pers. ex Fr.).
With field pea acreage increasing in the Peace River region, more research was
needed to identiQ the presesence and severity of pea diseases in the region. This
information dong with a better understanding of field pea diseases would allow acreage
to increase wi& minimal risk h m disease. Many crops now grown in the region require
a minimum 4-year rotation to prevent problems with diseases. Canola, for example, a
crop which is considered to be the highest returnùig cash crop in the region should only
be grown once every 4 years to prevent diseases such as sclerotinia stem rot and virulent
blackleg from becoming a problem (Harrison 199 1). Rotations could be planned with the
howledge that disease cycles are being broken and crops are benefiting from the
inclusion of peas in the rotation.
1.4 Objectives
The objectives of t h i s study were to:
Conduct a field survey to assess the incidence and severity of root rot and to
identiQ root and foliar diseases in peas grown in the Peace River region of
Alberta,
Conduct a production s w e y of pea growers in the Peace River region of Alberta
to determine if a correlation exists between any production practices and root rot
incidence and severity.
Use the results of the survey and a literature review of field pea diseases to
propose and develop a field pea disease management program for field pea
growers in the Peace River region of Alberta.
Chapter 2 - Pea Disease Descriptions
2.1 Soil-borne Diseases - Seedling Blight, Seed Rot & Root Rot
This disease complex is caused by several soil-borne fungi. Pythium ultimurn
Trow and other Pythium species, Fusannum soluni (Mart-) Sacc. Esp. phi (Jones) Snyd. &
H~IIs., F. oxysponim Schlecht. emend Snyd & Ham. fhp. pisi (Hall) Snyd. & Hans. and
Rhizoctonia solani Kahn are aU involved in this complex in Alberta. Aphanomyces
euteiches Drechs. f.sp. pisi Pfend. & Hag. and ntielaviopsis basicoln Berk. & Br.,
although reported as part of this disease complex in other pea growing areas of the world
are not presently lmown to occur in Alberta (Howard RJ. - Personal Communication
1998).
These f h g i cm attack individuaIly or collectively anytime fkom seed germination
to rnaturity of the pea plant. Together they cause a complex of seed rot, damping off,
seedling blight, root and foot rot diseases, the symptorns of which overlap and therefore
are difficult to differentiate in the field. Soil-borne diseases are considered the main
limiting factor in increasing and stabilizing pea yields in North Amenca (Kraft 199 1).
Each of these mot diseases will be descnied individually. Because these pathogens are
rareIy found individually in the field (Swamon et al. 1984; Hwang and Chang l989),
however, control recommendations will be given for the disease complex as a whole.
2.11 Fusarium Root Rot
Fusarium soIani (Mart.) Sacc. f. sp. pisi (Jones) Snyd. and Hans. is a serious
C 9
pathogen of peas in many parts of Canada and the US. Yield losses of 26 - 57% have
been reported in both dryland and irrigated areas (Basu et al. 1 976; Kraft 1 984; Hwang et
al. 1995a). Fu~arium spp. are the most common root rot pathogens isolated fkom peas in
Alberta (Surnar & Howard 1979; Hwang & Chang 1989).
Symptoms:
S ymptoms first appear on the hypocotyls, epicotyls and cotyledonary attachent
area. Reddish brown streaks initially develop on the primary and secondary roots and
eventudly coalesce to fom a dark reddish-brown colour on the primary root up to the
soil line. Graying, yellowing, necrosis of lower foliage, and stuntirxg can al1 occur if
infection is severe (Kr& & Kaiser 1993). Foliar symptorns often appear after periods of
w m temperatures and heavy r;tinfdl. Fusarium root rot is favoured by poor crop
rotations, high soil temperatures (22' - 30" C), moist soils, acidic soils (pH 5.1 - 6.2) and
low fertility (Kraft 1984; Tu 1994). Soi1 compaction by f m machinery has also been
shown to increase the incidence and severity of F u s a root rot (Tu 1994).
Disease Cycle:
Initial infection usually occurs through the stomata on the epicotyl and hypocotyl
and the pathogen spreads into the root system. Infection can also occur through direct
peneîmtion of the cuticular surface of the epicotyl. The primary suMval structures are
chlamydospores. Chlamydospore germination is triggered by the release of exudates as
pea seeds germinate (Kraft 1984).
2.12 Fusarium Wilt
Fusarium wilt of peas is caused by Fwurzùrn oxysponrm Schl. f. sp. pisi (Hall)
Snyd. & Hans. Several races have been identified with races 1,2,5 & 6 the most
important in North Amaica Race 2 is also hown as near-wilt. Races 1 and 2 are more
of a problem in eastern North America and races 5 and 6 in western North America
(Haglund 1984). Tu (1992) reported that although fus& wilt was not the moa
common disease of peas in southwestern Ontario, it was the most serious because of its
severity.
Symptoms:
Symptoms for race 1,5 & 6 are very sirnilar. Race 2 symptoms have some
variation. In mots when cut longitudinally, there is a yellowish, orange colour in the
vascular tissue, which can extend into the basal area of the stem. For race 2, the
discolouration is more pronounced and is dark orange to red in colour and extends to the
crown of the plant. M e r symptoms common to al1 races include dowward curling of
the leaves and stipules, thickening of the basal intemode and brittle leaves and stem.
Yellowing of the leaves progresses &om the base of the plant to the top of the foliage.
Plants often die due to loss of the root system (Haglund 1984; Tu I987a).
Disease Cycle:
Chlamydospores are the primary çunival structures of this fungus and can remain
infective in soils for 10 or more years. F. oxyspomm Esp. pisi can also be seed-borne.
Soi1 temperatures of 20 ' to 2 1 OC are considered optimal for infection by race 1,5 and 6
and 25 OC for infection by race 2. Chiamydospores germinate and infect pea plants
through the fibrous roots. Infection then progresses into the vascdar system. Race 1, 5
and 6 are most often found in small circular patches in fields. Race 2 is usually found in
plants scattered throughout the field (Haglund 1984). Resistant cultivars are available for
the different races (Cousin 1997).
2.13 Pythium Root Rot
Seed rot and seedling damping-off caused by Pythium species are considered a
major Iimiting factor in pea production in Alberta (Hwang et al. 1997) e t h i u m species
are common in Alberta soils. Hwang and Chang (1 98 9) found an average of 600
propagules (fimgal colonies) of q.thizrm species per gram of air dried soi1 in a survey of
soils in northeastern Alberta P. irregulare and P. ultimum have been the two species
most fiequently isolated fiom field pea in Alberta (Hwang S.F. - Persona1
Communication 1998).
Symptorns:
Seeds are rotted and when removed f?om the soil, emerge with a layer of soi1
around them. This layer of soil is full of whitish fimgal hyphae. Ernerging radicles or
plumules, if produced, may be soi? and watery and cotyledons may or may not be rotted
(Haman 1984a). Germinating seeds are only susceptible for 48 - 72 hours. Once the
radicle emerges korn the seed coat, the seed is no longer susceptible to infection. New
developing tissue, however, remains susceptible. Infection can occur at the tips of feeder
12
roots where juvenile tissue can be attacked and destroyed. This can lead to root pninuig
and/or reduction in root length. Depending on the severity of the infection, seedlings may
become stunted and chlorotic and collapse as the root base decays and hims tan to light
brown in colour (Hwang et al. 1997). This results in patchy stands in the field due to
missing seedlings. Infected plants tend to lack vigour and often yield poorly.
Disease Cycle:
Pylhium spp. are found in most soils as dormant sporangia or oospores. These
germinate very quickly in the presence of a sîimulus such as exudates fkom seeds. Seed
coat infection c m occur in as little as 6 hours and infection of cotyledons within 40 hours
(Hamm 1984a). Once infection occurs, large numben of sporangia and oospores are
produced. Infection is very dependent on seed quality. Cracked seeds le& more exudates
and are more likely to attract Pythium and be infected than intact seed. Severd studies
have shown that q>thiurn spp. are more of a problem when soil temperatures are cool,
soils are poorly drained and soil moisture is high (Benedict 1969; Kraft & Roberts 1969;
Hwang et al. 1997). Cool soil temperatures slow the rate of seed germination and make
them susceptible to infection over a longer period of thne (Leach 1947).
2.14 Rhizoctonia Root Rot
Rhizoctonia solani Kühn is endemic in most pea producing areas of Alberta and
can cause serious damage under the nght enviromenta1 conditions (Hwaog & Chang
1989). The AG2 and the AG4 groups have been the most fiequently isolated fiom field
pea in Alberta (Hwang S.F.-Personai Communication 1998).
Symptoms:
R . soloni prefers well-aerated soil near the surface and most often infects the
hypocotyl, epicotyl and seed (Kraft & Kaiser 1993). Symptoms on seedings k t appear
as water-soaked Lesions that eventually tum reddish brown. Death of the growing point
may occur as it emerges fiom the ground which can lead to multiple shoots emerging and
dying. Symptoms on older plants appear as reddish-brown sunken lesions on the epicotyl.
This can cause girding and often leads to stunted plants. Seedlings generaily become Iess
susceptible as they get older. Infection most often occurs close to the soil surface where
oxygen levels are high and this is where symptoms are most often found (Kraft &
Harman 1984).
Disease Cycle:
R. solani cm survive in the soil for many years as a saprophyte and only
decreases in number in the presence of non-host crops. Infection occ-irs when
genninating hyphae growing through the soil make contact with a susceptible host. An
infection cushion is fonned on the surface of the host and an uifection peg penemtes the
epidermal surface. Growth continues inside the host both inter and uitracellularly.
Infection normally occurs early in the life of the seedling and seedlings become less
susceptible with age. Overwintering occm as mycelium (Kraft & Harman 1984; Hwang
& Chang 1989). High soi1 temperatures (24'-30°C) are known to cause higher rates of
infection in pea plants ( K . & Kaiser 1993; Xi et al. 1995).
2.15 Aphanomyces Root Rot
World-wide, aphanomyces root rot, caused by Aphanomyces euteiches, Drechs.
f.sp. pisi Pfend. & Hag. is considered one of the most serious root rot diseases of peas
(Jacobsen & Hopen 198 1; Muehlchen et al. 1990; Parke et al. 199 1) because of its
resistance at attempts to find culmral, chernical and genetic controls (Pfender 1984). Ln
North America, it is a much more serious problem in the Great M e s and northeastem
States of the United States than in the western pea growing areas (Pfender 1984).
Aphanomyces root rot has been reported on peas in Manitoba (Grau et al. 1991) and
Ontario (Tu 1992). The presence o f aphanomyces root rot has yet to be confirmed in
Alberta (Howard R.J. - Persona1 Communication 1998).
Symptoms:
Symptoms can develop in as little as 7-14 days when inoculum levels are high
(Pfender & Hagedorn 1983; Kraft & Kaiser 1993). S ymptoms f i s t appear on roots as
straw-coloured lesions that gradually spread causing a sofi, watery rot close to the soil
line that is fairly distinctive. Darnage to the root cortex results in slmghing off of the
cortex when infected pea plants are pulled from the soil. This damage also Ieads to
yellowing from the bottom up and stunting of the plant as it is unable to extract nutrients
and moisture from the soil (Pfender 1984)-
Disease Cycle:
Aphanomyces root rot c m infect peas at any growth stage but occurs more
cornrnonly at thz seedling stage (King & Parke 1993; Kraft & Kaiser 1993). Infection on
pea roots occurs when oospores germinate via genn tubes. Mycelium then grows
15
throughout the root tissue causing degradation of the root cortex. Oogonia and antheridia
are produced which leads to the formation of mature oospores in 7 - 14 days. These are
dispersed in the soi1 as the roots decornpose and are present for reinfection when host
plants are next grown (Pfender 1984). Oospores are very persistent in the soil.
Recommendations for reducing inocdum to safe Ievels range fiom 6-8 years (Temp &
Hagedorn 1967), to 9 years (Pfender & Hagedorn 1983), to over 10 years (Pfender 1984)
without peas.
2.16 Thielaviopsis Root Rot
Thieimriopsis h i c o l a Berk & Br. is considered a senous root rot pathogen of
peas in some pea growing areas of the United States. Although reported in Ontario, it is
not yet considered a problem in Canada (Tu l987a).
Symptoms and Disease Cycle:
Thielaviopsis root rot is aiso known as black root rot because of the necrosis of
the cortex it causes on both tap and lateral roots. Chlamydospores are produced which
germinate in the soil to infect new plantings of peas. T. basicola often occurs with F.
solani and together they cause a necrosis and blackening of the entire mot system.
Occasionally, small, black lesions also occur on the lower leaves of infected plants. High
soil temperatures (28 OC) favour this disease (Lloyd &- Lockwood 1963) but severe losses
have been reported at 20°C. Heavier soils also seem to be prefened by this pathogen
(Blume & Harman 1979; Haman 1984b).
16
2.17 Prevention and Control of Seedling Blight, Seed Rot and Root Rot:
Since al1 of these root rot pathogens are rareiy found individually in a fie14
control measufes should be aimed at controhg the complex of pathogens. Since
aphanomyces and thielaviopsis root rots are not known to occur in Alberta at present,
these control recommendations will focus mainly on the other root rot pathogens known
to occur in Alberta.
The most effective control strategy for these soil-borne diseases is a 4-5 year crop
rotation away fiom peas or other susceptible hosts such as dfalfa and clovers.
The use of seed with high vigour is another very important way to avoid problerns with
these fun@ (Kraft & Kaiser 1993). High vigour seed genninates and emerges quickly,
thus decreasing the time the pea plant is susceptible to infection. Vigour tests for peas are
available from selected seed testing labs and should be used to select pea seed with high
vigour.
Good soi1 fertility will also shorten the time that the pea plant is in a susceptible
growth stage. Peas inoculated with the correct strain of rhizobia will produce most of the
nitrogen they require but adequate amounts of phosphorus, potassium and sulphur are
needed to ensure that peas get off to a good start. Peas need to have adequate nutrients to
produce healthy plants that are able to resist attack by various soil-borne and foliar
pathogens.
Liming of acid soik is another way to reduce root rot. Tu (198%) showed that
soi1 pHs between 6.0 and 6.4 Ied to the lowest level of root rot. Soi1 pH lower or higher
than this range had higher levels of root rot.
Reducing soi1 compaction has also been shown to decrease the incidence of root
rot (Ex 1994). This can be accomplished by subsoiling to break up the compacted layer
of soi1 below the depth of cultivation. Compaction can dso be reduced by the adoption of
minimum and zero-tillage which involves fewer heavy machinery passes on the field.
Thulim ( T a 75 WP) and captan (Captan FL) are registered for control of seed
decay, damping off, root rot, and seedling blight. Metylaxyl (Apron FL) is registered for
control of seed rot and seedluig blight specific to Pythium species and can be mixed with
thiram to give a broader control spectnun (Anonymous 1998a). These hgicides,
however, offer no protection once the plant is past the seedlhg stage.
A biocontrol agent, ACM94 1, for root rot is presently being tested in western
Canada and is showing controi levels similar to thKam (Xue 1996).
Herbicides such as (4-chloro-2-methylphenoxy) acetic acid (MCPA), 4-(4-chloro-2-
methylphenoxy) butanoic acid (MCPB) or a mix of MCPA and MCPB (Tropotox Plus)
should be avoided as it has been shown that they cause stress on the plants and
consequently increase root rot severity (Tu & Hamil1 1986; Tu 1992). Many effective
herbicides are presently registered for use on field peas that do not cause stress on the
plants (Anonymous l998a).
2.2 Foliar Diseases
This disease complex is caused by three different species of fungi, each of which
produces a clearly recognizabl e symptom: MycosphaereZZu pinodes (Berk. & B lox .)
Vestergr. (the pdect stage of Ascochyta pinodes) which causes blight; A. pisi Lib. which
causes leaf and pod spot and Phoma rnedicaginis var. pinodella (Jones) which causes foot
rot (Wdlen 1974; Lawyer 1984a; Kraft 1991). M.pin0de.s is the most common on field
pea in western Canada (Wallen et al. 1967; Berkenkamp & Kirkham 199 1; Rashid et al.
1994; Xue & Burnett 1994; Xue et al. 19%) and the disease it causes is ohen called
ascochyta blîght (Warkentin et al. 1 996) or mycosphaerella blight (Anonymous 1 997a).
Average yield losses due to mycosphaerella blight on field pea in Westem Canada
are estimated to be 10 percent. Yield losses of up to 80 percent have been reported in
Westem Canada when a heavy infection occurs in mid-June (Anonymous 1997a) and up
to 50 percent on processing peas in eastem Canada (honymous 1996b; Martens et al.
1984).
The three species are grouped together as a cornplex because they often occur
together and distinguishing between them in the field is difficult. Al1 three pathogens are
common throughout the temperate pea growing areas of the world, such as Australia,
New Zealand, North America and Europe (Krafi 199 1). M. pinodes has even been
reported fkom subtropical regions in Central and South Arnerica, Afiica and Haiti
(Lawyer 1984a).
Symgtoms:
These fùngi attack roots, leaves, stems, flowers and pods. S ymptoms of infection
by A. pisi are different âom those of M. pinodes or P. m e d i ~ a ~ n i s var. pinodella.
Lesions caused by A. pisi are partially sunken, tan in colour and surrounded by a well-
defmed dark brown margin. Circular lesions are found on pods and leaves while lesions
are more elongate on stems (Skolko et al. 1954; Lawyer 1984a). Numerous pycnidia are
usually found in the lesions. Lesions of A. pisi are rarely found on any plant parts below
the soi1 line. (Lawyer 1984a)
It is much more difficult to Merentiate between lesions caused by M. pinodes
and P. m e d i ~ a ~ n i s var. pinodella without a detailed laboratory examination. However,
the assumption can usually be made that if the lesions on leaves, stems and pods are
widespread and severe, M. pinodes is the cause whereas if the lesions are severe on
subtemean parts, P. rnedicaginis var. pinodellu is the cause. M. pinodes produces small,
brown to purplish, irregular flecks without definite margins that initially appear on pods,
leaves, stems and the cotyledonary area (Skolko et al. 1954). These lesions enlarge if
weather conditions are fàvoufab1e. Optimal temperatures for disase development by M.
pinodes are between 15 O and 18 O C (Wallen et al. 1967). Dark-brown to black pycnidia
are eventually produced and as the lesions enlarge, a distinct concentnc tan and brown
ring pattem is formed This is ofien more pronounced on leaves and pods than on other
plant parts. Lesions developing on stems tend to form long, wide purple to bluish-black
streaks that eventually codesce and may completely girdIe the stems, pedicels or tendrils
(hwyer 1984a). These streaks are more common near the nodes and on the lower
20
portion of the stem (Skoko et al. 1954). Pod infection can lead to seed iafection if it
occurs early enough. Seed infection may show no visible symptoms if infection is light;
if severe, vaqhg degrees of shrinkage and discolouration can develop (Anonymous
19960). Under drier conditions, the concentric ring pattem of the symptoms is less
pronounced and may show up only as a uniform yellowing of lower leaves. If the
blossom becomes infeste4 girdiing of the sepal often occurs leading to pod drop or
distortion. Bretag et al. (1995) showed that in southem Australia, yield was closely
correlated to disease severity. For most pea varieties evaluated in their study, there was a
5 to 6 percent yield reduction for wery 10 percent of the stem area affected by ascochyta
blight.
Disease CycIe:
These pathogens can be seed, stubble or soil-borne. Seed-borne infection by M.
pinodes is considered to be the primary source of inoculum in virgin pea fields (Xue et
al. 1996). M. pinodes also competes well with other soi1 micoflora and survives as
thickened mycelia (sclerotia), chlamydospores and pycnidia on pea straw hgments and
in the soil. When moisture and temperature conditions are favourable, residual pycnidia
mature, new pycnidia develop and spores are released to infect new plants.
Pycnidiospores are produced throughout the growing season during periods of moist
weather and cm be carried by rain splash to healthy plants. Ascospores are also produced
which can be carrïed by whd for a kilometre or more (Martens et al. 1984). P.
medicnginis var. pinodellu zlso produces chlamydospores and pycnidia and is also quite
persistent in the soil. A. pbi. on the other han& competes vay poorly with other
21
microflora in the soi1 and overwinters very pooriy. The main source of infection fiom A.
pisi is fkom seed-borne spores. Symptoms of M. pinodes and P. medicagink var.
pinodeh appear in 2 to 4 days. A. pisi symptoms appear in 6 to 8 days. Under optimal
moisture conditions the pathogen cm spread rapidly in a field.
Prevention & Control:
Pea seed should be examined for Ascochyta presence by an accredited seed Iab
and only pathogen-fkee seed should be used. If disease fYee seed cannot be found, seed
with as low a level of Rscochyta as possible should be used and the seed treated. Seed
treatrnent will help but only if seed is used on virgin pea land (Wallen et al. 1967).
Thiram, a mix of thiram and iprodione (Rovral) or a mix of metdaxyl (Apron FL),
carbathiin and thiabendazole (Crown) has been shown to give excellent control of seed-
borne M. pinodes on peas (Rhashid et al. 1996). Only thiram, however, is registered in
Canada for use on peas (Anonymous 1998a).
Chlorothalonil (Bravo 500) has recently been registered for use on field peas in
Western Canada to control mycosphaerella blight. Chlorothalonil is applied in the early
btoom stage and can be repeated up to three times at 10 to 14 day intervals if conditions
for disease development are favourable (warm and humid) (Anonymous 1998a).
BenomyI (Benlate) has also been shown to provide good control of this disease
(Warkentin et al. 1996) but is not registered for use on field peas in Alberta at this time
(Anonymous l998a).
Crop rotations done do not provide effective control of this disease cornplex
since ascospores can travel long distances by wind (K& 1991). Care should be taken to
22
avoid seeding next to any previous year's pea fields.
Plowing or discing to bury all crop residue as soon as possible &er harvest will
also help to prevent the fungus f b r n being dispersed by wind and min.
2.22 Sclerotinia Rot
Sclerotinia rot in peas, caused by the fungus ScZerotinin sclerotionrm (Lib.) de
Bary, has been reported fiom Canada, the United States, Morocco, Bermuda, New
Zealand, Brazil, Argentins, the Netherlands and Scotland (Huang & Kokko 1992). It is
often cailed white mold or pod rot in peas. This disease is common in canola fields in
Western Canada and has recently started showing up in pea fields. niere is defmitely
concern about sclerotinia rot when peas and canola are grown in the same rotation.
Davies (199 1) reported a higher incidence of sclerotinia stem rot in canola when peas
were included in the rotation. If conditions are favourable and infection is early,
sclerotinia rot can cause serious yield losses in peas.
Symptoms:
The later stages of plant development, from f i owering to maturity, is when the
disease occm most often. Dense canopies and high humidity are favourable to outbreaks
of sclerotinia. The fmt sign of infection is the appearance of a light brown to brown
discolouration on the stem, leaves or pods. With pod rot, this occurs at the base of the
style with a small water-soaked lesion appearing on green tissue at the end of the pod
(Huang & Kokko 1992). With stem rot, the symptoms are more common in the node
23
area. Whitish mycelial mats develop over the affected areas that over time cause the
tissue undemeath to turn soft and decay (Lawyer 1984b). Black, hard bodies known as
sclerotia form in the mat and within the stem and pods. AfZected plants often appear
wilted and ripen premahlrely due to rotting of the stems. Lodging is common in afEected
areas.
Disease Cycle:
S. sclerotiorum will grow over a wide range of temperatures eom O O to 28 O C.
Optimal temperatures, however are between 20 O and 25 O C . Lawyer (1 984b) descnbes
the mycelium as hyaline, septate and branched. The mycelium forms sclerotia that vaiy
in size and shape depending on environmental conditions and age. Sclerotia consist of a
layer of pigmented external cells, three to four layes thick, m a h g them hard and
resistant to decomposition. When humidity is hi& and moisture is abundant such as
within a thick canopy, tan, fleshy, golf-tee shaped apothecia emerge f?om the sclerotia.
Numerous ascospores develop within the apothecia and are released into the air to infect
nearby plants. Ascospores cannot infect pea plants directly and need a source of
senescent fissue such as stamens or flower petah to germinate (Huang & Kokko 1992).
Following germination, hyphae f o m that spread and fonn numerous infection cushions
that penetrate the plant by mechanical pressure (Lawyer l984b; Huang & Kokko 1992).
The fungus overwinters as sclerotia either in plant debris or in the soil. infection
cm occur by one of two methais. Direct infection by mycelia c m occur fiom sclerotia
located in or on the soil surface or fiom ascospores released from apothecia Airborne
ascospores are the most common method of infection in peas (Huang & Kokko 1992).
Ascospores cm be released over a considerable period of t h e if conditions are
favourable and can be dispersed by air currents for several kilometres to infect
neighbouring fields. The critical period of infection, however, is during the flowering
stage (Hamson 199 1). Under moist conditions, ascospores land on senescent stamens,
filaments or petals that become colonized by threadlike running hyphae. These hyphae
spread fkom the senescent tissue to healthy tissue and fomi numerous infection cushions.
Peneîration occurs at the bottom layer of hyphal celk of each infection cushion. Pollen
grains are another source of nutnents for the hyphae to colonize. Lesions appear 4 to 5
days afier infection and within 8 days, dark to p y i s h brown lesions ranging nom 1 to 5
mm cm be seen on the infected tissue (Huang & Kokko 1992). When the peas are
harvested, sclerotia are either harvested with the seed or fall to the ground with the vines.
Only sclerotia in the top 6 cm of soil c m produce apothecia. Sclerotia buried deeper
remain donnant and c m survive for 5 to 7 years in the soil untiI favourable conditions
exist for apothecia formation (Lawyer l984b; Harrison 199 1).
Prevention and Control:
A 4 to 5-year crop rotation out of susceptible crops is recommended to prevent
possible infection. Sclerotinia has a hown host range of 360 plant species and
susceptible crops incliide canola ( h s i c a mpus & B. rupa), d o w e r s , beans, alfalfa,
clover and potatoes. Common weed hosts include stinkweed (Thlaspi anense L.), hemp
nettle (Galeopsis tetruhit L.), thistles (Cirsium anense L. & Sonchus arvensis L.),
shepard's-purse (Capsella bursa-pasto* K.] Medic.) and wild mustard (Brassica kaber
[D.C.] Wheeler var. pinnat3da [Stokes] Wheeler) (Harrison 1 99 1). Moldboard plowing
25
for deep burial of sclerotia can prevent the sclerotia fkorn germinating. Planting semi-
leafless varieties will help to create a more open canopy that is less likely to have the
high rnoishire needed for infection to occur. Fungicides such as benomyl (Benlate) and
iprodione (Rovrai) are used in canola to control the disease (Harison 1 99 1 ). However,
neither is registered in Canada for use on field pea at this tune (Anonymous 1998a).
Biological control of sclerotinia rot in peas may be an option in the not-too-
distant future. A field trial is presentiy being done at the Lethbndge Research Station to
study the efficacy of an application of antagonistic bactena to peas to control sclerotinia
rot (Huang et al. 1993).
2.23 Powdery Mildew
This disease is caused by Blumeriapisi f.sp. pisi DC. and occurs worldwide
wherever peas are grown, generally in late mahiring crops, and is often considered of
little economic importance @ixm 1978; Zimmer 1984). It shows up as clouds of dust
when the peas are swathed or combined. Yield and quality of the pea seed itself are
usually not affecteci. In yean when the disease occurs early, however, it can cause
serious yield and quality losses.
Symptoms:
Symptoms first develop as off-coloured spots on the upper surface of the lowest
and oldest laves, then rapiciiy spread to cover the entire surface of leaves, stems and
pods with a fine, powdery, bluish-white mildewy growth. Undemeath the mildew, the
26
tissue often tums purplish in colour. As plants age, tiny pinhead shed fniiting bodies
(cleistothecia) develop. These are brownish in colour at kt, eventually tuming black at
maturity. Infection can also reçult in withering of foliage. If infection occurs on the pods,
penetration of the pod may occw and cause the seed to turn grayish-brown in colour. If
infection is severe, it can result in hollow se&. Other effects of severe infection are
reductions in plant weight, weight of pea seed, number of peas per pod and pods per
plant, as wdl as plant height (Dixon 1978; Reiling 1984a). Nodulation and nitrogenase
activity are also known to be reduced by Blumeriapisi f.sp. pisi. The earlier the infection
occurs, the greater is the reduction (Singh & Mishra 1992).
Disease Cycle:
BZumeriapisi fsp. pisi is an obligate parasite which obtains nutrients fkom the
plant through haustoria in epidermd cells (Agrios 1988). The cleistothecia oveNvinter in
plant debris and on other hosts. The disease can also be seed-borne (Dixon 1978). As
temperatures increase in the spring, the cleistothecia release ascospores that are c h e d
by wind cwents to healthy susceptible plants. Within 2 hours, under dry conditions
conidia gemiinate to produce a single germ tube which forms an appresso~um. Infection
pegs develop fiom the appressorium, pierce the ce11 wall and form a haustorium within
the cell. The first hypha to develop grows fiom the surface of the conidium followed by
second and third hyphae. Subsequent hypbae grow from the appressorium (Smith et al.
1996). Appressoria develop at regular intervals dong the hyphae. Conidiophores also
develop on hyphae as they spread over the leaf surface (Falloon et al. 1989).
Powdery mildew is seldom a problem until late in the season when conditions of
high humidity without rainfall are more common favouring development of the disease.
S u m e r rahs damage the conidia causing them to burst instead of genninate.
Cleistothecia are not affected by min but usually develop in the fa11 and contribute to the
spread of the pathogen. In late summer or early fa& when dews occur without rain the
disease cm spread across a field of peas very quickly (Martens et al. 1984). Fomuiately,
most peas have rnatured suficiently by this tirne that the pathogen causes little if any
yield or quality losses.
Prevention and Control:
Seeding early (Zimmer 1984), use of earlier maturing varieties, crop rotation and
burial of crop residues al1 help to reduce uifection by powdery mildew.
Kumulus DF, an 80% sulfur product has recently been registered to control powdery
mildew on peas in western Canada (Anonymous 1998b). Varieties have also been
developed that are resistant to powdery mildew (Warkentin et al. 1995; Cousin 1997).
2.24 Downy M d e w
This disease caused by Peronospora viciae @&) Casp., is favoured by cool,
wet growing conditions and occurs most ofien on early seeded crops.
Symptoms:
Symptoms c m be expresseci either systemically, tocally (on leaves, tendrils and
flowers) or as pod infections. Systemic uifections produce the most severe effects on the
pea plants resulting in stunting, distortion and the proliferation of white, cottony growth
28
on the plant. Profuse sporulation of the fungus occurs on the surface of the plant. Locd
infections appear as fiuffy, white to bluish, cottony patches on the undersides of leafiets.
The upper side of the kaflets becomes yellow and dies. Plants ofien die before fiowerinp
or if pods are produced, they are flatteneci, yellow, distorted and rarely set seed
Pod infection f h t appears as blotches which become brown with green areas inside
them. This causes small, brown, sunken spots to occur on the seeds which if swere can
lead to seed abortion. A cottony growth cm be visible inside the pod that contains
numerous oospores. Plants often develop resistance as they age and new infections may
only appear as çpotting on the leaves (Dixon 198 1; Reiling I984b).
Disease Cycle:
The pathogen overwinters primarily as oospores in the soil and on plant debris.
Longevity of oospores in the soil can be as long as 15 years. Seed-borne infection cm
also occur and often causes seed to fail to gemiinate. Seedings can become infected
systemically or locally from soil-bome oospores. Sporangiophores originate fiom the
mycelium, which produce sporangia and are the main source of secondary infection on
stems, pods and Ieaves. They genninate to produce a germ tube that penetrates through
the stomata and produces a haustorium. The pathogen progresses intercellularly. Spore
dispersal and infection occur mainly through &dl. Cool, moist weather favours the
diseaçe. Spore survival decreases with inmeashg temperatures and decreasuig humidity
(Reiling 1 W b ) .
Prevention and Control:
A 2 to 3-year crop rotation will control this disease very well. Deep burial of crop
29
residue can help prevent spread of the pathogen to a subsequent crop. Use of disease-fiee
seed is also very important to prevent seed-borne infection. Resistant varieties are
presently behg developed Metalaxyl (Apron FL) has been shown to provide some
protection against soi1 and seed-borne infections (Reiling 1 984b)-
2.25 Gray Mold
This disease is caused by BohytiF cinerea Pers. :Fr. and occurç under humid
conditions in areas of North Amenca, New Zealand, Japan and Europe (Lawyer 1 984~).
Gray mold can cause a rot on pods, stems and leaves on pea plants or on fiesh peas and
pods in m i t or storage.
Symptoms:
Funy, gray lesions are the h t symptoms that appear on lower areas of the stem.
Under humid conditions, the fungus spreads until the entire lower foliage appears a
fuuy, gray colour. The fûzziness and gray colour corne fiom the abundance of conidia
produced on the leaves and stems. As the disease progresses, leaves become shrivelled
and dry and defoliation of the lower leaves occurs. Pod infections cause the most
damage. Clinging blossoms provide a humid enviromnent fiom which the tip of the
young pod can be infected. Small, oval, water-soaked lesions develop and spread up the
pod. These lesions are tan at f k t but him grayish with age and ofien develop sclerotia
which appear as small black specks.
Peas in transit or storage develop water soaked, grayish green lesions that quickly
30
become covered in a grayish, white mycelial mat. The infection spreads quickly and
causes pods and the seed h i d e to rot (Lawyer 1984~).
Disease Cycle:
B. cinereu overwinters as smail, thin, black sclerotia Mycelium and spores can
also serve as overwintering stages but are not as resilient as the sclerotia Infection occurs
quickly under optimal conditions of 16 " to 2 1 OC and 100% humidity. Dispersal of
conidia c m occur through rain splash, by wind or by irrigation water (Lawyer 1984~).
Prevention and Control:
The use of potassium fertilizer in potassium deficient mils has been show to
reduce the severïty of gray mold (tawyer 1984~). Systemic fùngicides such as benomyl
will provide some control but no chernicals are presently registered for the control of
gray mold on peas in Alberta (Anonymous 1998a).
2.26 Anthracnose
Anthracnose of p a s is caused by ColZet~~chumpisi Pat. It is considered to be of
minor importance since infections are usually small and isolated (Hagedorn 1 %Ma).
Symptoms:
Lesions due to anthracnose can occur on foliage, stems and pods. Pod lesions are
circular, sunken, with reddish to brown margins and reddish coloured centres. Close
examination of pod lesions o h reveal hi t ing bodies called acervuii containing orange-
pink spores masses. Leaf and stipule lesions are oval with brown margins and grayish
3 1
centres. Stem lesions are elongate and appear copper-coloured when moist and grayish
when dry. Severely diseased plants have an overall reddish-brown appearance (Jones &
Vaughan 192 1 ; Hagedorn l984a)-
Disease Cycle:
C. pisi can be seed or soil-borne. The pathogen overwinters in plant debris and
this is the most common source of inocdum. Rain splash spreads conidia to the surface
of leaves and stems where they germinate to form a germ tube that produces an
appressonum from which an infection peg penetrates the pea plant. Warm temperatwes,
high humidity and recurrent rains al l favour development of this disease (Hagedorn
l984a). Aathracnose is commonly found occming with Ascochyla spp. and behaves
more like a secondary pathogen colonizing lesions causeà by the Ascochyta spp. (Ou &
Walker 1945).
Prevention and Control:
Use of disease fkee seed and crop rotation with non-leguminous plants will
prevent problems with this disease.
2.27 Aiternaria Blight
Altemaria alternata (Fr.:Fr.) Keissl. is the causal organism of this disease.
Although symptoms c m be damaging to a pea plant, this diseaçe is not considered to be
of economic importance because of its infiequent and isolated occmence (Susuri et al.
1982).
Symptorns:
Foiiar lesions due to A. altemata are oval with a concentric ring pattern inside,
tannish-brown in the centre and lighter towards the rnargins. Up to half or more of the
leaves and stipules c m be covered by lesions when an outbreak occurs. Pod lesions are
smaller, raised and brownish in appearance. Slight stunting of pea plants can also occur.
Prolonged high humidity (3 or more days) and warm temperatures (16 O - 24°C) favour
development of altemaria blight (Susuri et al. 1982; Lawyer l984d).
Prevention and Control:
Rotation with non-cmciferous crops reduces air-borne inoculum nom plant debris
and is the best means of prevention (Martens et al. 1984). No resistant varïeties of peas
are known.
2.28 Septoria Blotch
Septona blotch, caused by Septoriapisi Westend., occun mainly on senescent
foliage, pods and stems of peas. As such, it generally causes little quality or yield loss
and is not considered of economic importance (Hagedorn 1984b).
Symptoms:
Lesions are blotchy in appearance with no distinct margins and develop rnainly on
the lower, older leaves, pods and stems of the pea pIant. Early formed blotches are
yellowish-green and eventually darken and enlarge and are hegular in size and shape.
Entire leaves or stipules may be covered by blotches which coalesce. Black pycnidia
33
develop profusely over the surface of the affected areas (Zaumeyer 194 1 ; Hagedorn
i984b).
Disease Cycle:
S pLsi can be either seed or soil-borne, although seed transmission is not
considered important. The pycnidia on crop debns rel ease pycni diospores which are
spread by water splash to infect nearby plants. Temperatures of 21 O- 27 O C and high
humidity favour the development of this pathogen (Hagedorn 1984b).
Prevention and ControI:
The best control measure to prevent problems with this disease is a rotation to
non-leguminous crops to allow crop debris to decompose.
2.29 Cladosporium Blight
This is a disease most commody found in areas of high humidity. It is caused by
CZudosporium cladosporiodes (Fresen.) De Vries fkp. pisicoh (Snyd.) de Vries and is
often referred to as "scab" (Lawyer 1984e).
Symptoms:
Inward curling of leaves as they unfold is one of the first symptoms of infection
by C. pisicolum. Leaflesions can be irregular or circular in shape, are gray at fust and
evennially tum a tan to brown colour. Lesion margins-are thin and dark brown in colour.
Shredding of leaves can occur as necrotic areas fdl out or tear fkom wind. Brownish-
black lesions develop on most o h plant parts except for the pods and cadkers often
34
develop as these lesions age and crack Infected pods are often poclmiarked and have a
bumpy feel due to dark irregular pimples occuning on the pods. Seed infection can result
if pods are penetrated. Severely infected seeds develop black, circula spots with well-
defhed borders. A Iight infection may show up as light scattered black spots on the seed
(Lawyer 1984e).
Disease Cycle:
C. pisicoZum c m be either seed or soil-borne. Infection occurs under conditions of
high humidity and temperatures of 16" - 21 OC requiring three to seven days. n i e disease
is only known to occur on Pisum species and is more prevalent on young plants that are
actively growing (Lawyer 1 984e).
Prevention and ControI:
Use of disease-fkee seed, a two-year crop rotation and avoidhg low spots prone to
fog for planting are the main control recornmendations.
2.210 Black Leaf
Black leaf of peas is caused by Fusicladiurnpisicola Linford and is not
considered to be of economic importance because susceptible varieties are no longer
grown (Laver 19840.
Symptoms and Disease Cycle:
Small, white spots on the underside of leaves are the fkst symptoms, and become
sunken as they enlarge, to fom a concentric ring pattern progressing fiom tan coloured in
35
in the centre to gray to black at the edges. Lesions eventually darken as conidia form on
the underside of the leaf and eiongated Iesions may streak the Ieaves starting with the
lower leaves and progressing upwards. Blackening is due to closely packed conidia on
the leaf surface. Bottom leaves may eventually turn totally black, dry out and become
shredded in appearance (Lawyer 1984f).
Prevention and Controk
Use of resistant varïeties is the best control strategy.
2.3 Bacterial Diseases
2.3 1 Bacterial Blight
Bacterial blight caused by Pseudomonas syringae pv. pisi (Sackett) Young, Dye
& Wilkie is a senous disease of peas throughout the pea growing regions of the world
(Clarke 1990; Roberts et al. 1995; Hollaway et al. 1996).
Symptoms:
Shiny, water-soaked lesions initially appear near the nodes and stipules. These
spread to the stems, peduncles and tendrils and become darker in colour. The underside
of leaves and stipules develop water-soaked spots that appear dark green to brown on the
upper surface. With age, the spots deveIop an angular shape with dark margins and a
light coloured, papery appearance in the centre. Lesions appear translucent when held up
to the light (Lawyer 1984g). Lesions on pods are also water-soaked at kt, and him
darker as they mature and become sunken. The suture area is often a site of infection and
36
infected seeds may or may not show injury symptoms. Watery, dark spots sometimes
appear on the seed but more commody no visible symptoms appear on the seed. The
earlier the infection, the more senous is the damage to the pea crop. Seedlings attacked
by P. m g a e pv. pisi rnay not survive.
Disease Cycle:
P. syrirzgae pv. p l i ovefwinters predominantly as a seed-borne pathogen (Skoric
1927; Roberts 1992; Roberts 1993; Hollaway et al. 1996). The seed both extemaily and
intemally is covered by a dry, white f h of bacteria that is invisible to the naked eye
(Skoric 1927; Lawyer 19848). Seed can rernain infected for up to three years. Infection
occurs as the plumule contacts the infected seed coat during germination. The 3 lowest
stipules are the most cornmon infection sites. Soi1 moisture plays a role in the infection
process. Optimal temperatures for P. syringae pv. pisi growth are 26" to 28 OC with a
minimum of 3 OC (Lawyer 1984g). The higher the soi1 moisture, the higher is the rate of
infection (Skoric 1927; Wark 1954; Roberts 1992; Hollaway et al. 1996). Injus, nom
hail, fiost, wind, animals or machinery predisposes the plants to infection. Secondary
spread occws by rain splash, wind, machineiy, animals and irrigation (Lawyer 1984g).
Four races of P. syn'ngae pv. pisi are presentiy known and cultivars resistant to each race
are available.
Prevention and Control:
Since the pathogen is predominantly seed-borne, the use of disease-fke seed is
the primary method for avoiding the disease. Planthg resistant cultivars wiU also help to
reduce the impact of this disease. Seed production should be limited to more arid areas
and irrigation should be avoided.
Machinery should be disinfecteci between fields and spraying of pesticides should
be done by air instead of using ground applicators to avoid mechanical injury.
2.32 Brown Spot
This bacterial disease of peas is caused by fieudomonas syrfngae pv. syringae
van Hall, and aithough not as important as bacterial blight, can cause senous damage
under optimal environmental conditions (Lawyer 19848).
Symptoms:
In the field, symptoms of brown spot are difficult to distinguish from those of
bacterial blight. Under laboratory conditions, however, it is easier to tell the two diseases
apart especiaily at the seedling stage. Initially, the symptoms are small, water-soaked
lesions on leaves, petioles and stems which eventually tum a tan colour and take on a
bunit appearance. Stem lesions are sunken and tend to elongate upwards and ofien cause
stems, petioles and growing points to become distortai Infected leaves eventually dry up
and fdl off. Ki@ humidity is needed for the disease to become a senous problem
(Lawyer 1984g).
Disease Cycle:
P. syringae pv. syn'ngae c m be seed or soil-borne. It is generally only found on
the surface of the seed coat and as such does not survive for as long a period on the seed
as P. syringae pv. pisi. This pathogen does, however, have greater sunivability in the
38
soi1 than P. synngae pv. pisi. Optimum temperature for this pathogen is 24°C.
Othenvise, the disease cycle is much the same as bacterial blight (Lawyer 19848).
Prevention and Control:
Control strategies for brown spot are simila. to those for bacterial blight The use
of disease-fkee seed is the best way to prevent the disease fiom o c c M g . With brown
spot, however, s t o ~ g the seed for one year prior to planhg should be enough to
virhially elhinate the pathogen. Other known hosts of this pathogen (ie. dry beans
(Phaseolur vulgaris)) should also be avoided the year before or after peas (Lawyer
l984g).
2.33 Pink Seed
This bacterial disease is caused by Erwinia rhupontici (Miller) Burkholder and
was found for the first time on peas in southem Alberta in 1988. It now occurs fkequently
in southem Alberta and is often isolated f?om seed cleaning facilities (Huang et al. 1990;
Huang 1991).
Symptoms:
Tan to dark-brown lesions develop only on pods. Seed tums a pink colour and is
often shrivelled. Pink seed is more common in imgated pea fields than in dryland fields.
High humidity and mechanical injury fiom irrigation may d o w the bacteria to penetrate
hough wounds to infect the pea plants (Huang et al. 1990).
39
Disease Cycle:
Bactena enter pea pods through wounds caused by insects or mechanical injury
and infect the seed inside the pods. Quaiity of the seed is afEected more than the yield
(Huang et al. 1990).
Control:
Control methods have yet to be developed for this bacterial disease. Use of
disease-free seed should help to reduce the incidence of this disease. Cleaning between
seed lots in seed cleaning fadities may also help to reduce spread of the pathogen.
2.4 Viruses
There are more than 50 viruses known to infect peas worldwide (Hampton 1984).
Of these, only a few occur in Canada and only rarely is damage serious in western
Canada due to the severe winters and short growing season (Slinkard et al. 1994a).
There has never been a serious Wal disease outbreak on peas reported in Alberta
(Howard RJ. - Personal Communication 1998). As such, only a brief description will be
given of some of the main virus problems occurring in the United States, particularly in
the Pacific Northwest, an area that is geogmphically close to Alberta and where field pea
has been grown for many years.
The main vinises considered to be of economic importance in the USA are pea
seed-borne mosaic virus, pea enation mosaic vins, bean (pea) leaf roll vins, pea streak
virus and red clover mosaic virus ~ u s i n g pea stunt.
40
Other vinises reported on peas in the United States and Cana& are bean yellow mosaic
virus, alfalfa mosaic virus, cucuinber mosaic virus, watermelon mosaic vim 2, lettuce
mosaic virus, turnip mosaic virus, plantago mosaic virus, white clover mosaic virus,
clover yellow mosaic virus, clover yellow vein virus, cowpea aphid-borne mosaic virus,
desmodium yellow motîle virus, peanut rnottle Wus, peanut shint virus, soybean mild
mosaic virus, soybean mosaic vinis, broadbean wilt virus, clover blotch virus, pea dwarf
mosaic virus, pea early-browning virus, red clover mottle vhs, red clover necrotic
mosaic virus, and tomato black ring vinis (Hampton 1984). Tomato spotted wilt virus has
been reported on peas grom in a greenhouse in Manitoba (Zimmer et al. 1992).
2.41 Pea Seed-borne Mosaic Virus (PsbMV)
PsbMV was first detected in Canada in 1974 when breedulg lines of peas fiom
Morden and Saskatoon grown in winter increase plots in Califomîa caused an outbreak of
the disease in USDA lines also grown at the same location (Chico & Zimmer 1978;
Hamilton 1997). It is the only seed-borne virus occunfng on peas that is considered of
economic importance in North Amenca.
Symptoms:
Symptoms include stunting and downward curling of leaflets on seedlings shortly
after emergence. Leafmosaic, rosetting, shortening of intemodes and stunting may or
rnay not be present. Pod set may be affected in some cultivars or distorted pods may
develop with seeds that have split seed coats. Delayed maturity may also occur. Some
41
varieties may be infected and show no visible symptoms. (Stevenson & Hagedorn 1970;
Hampton 1984). Chico & Zimmer (1978) showed that yields of the varieties Trapper and
Centuy were reduced by 1 1 and 36 percent, respectively, when infected with PsbMV.
Disease Cycle:
PsbMV is a member of the potyvvus group that is seed-borne and transmissible in
a stylet-borne marner by aphids (Gonzales & Hagedorn 1971). Infected seed has been the
primary source of dissemination of this virus worldwide. In the field, the pea aphid
(Acyrthosiphonpisum L.) is the main vector of the virus spread from infected to healthy
plants, although other aphids are known to transmit PsbMV as well.
Prevention and Control:
The virus is not known to persist in infected plant hosts but survives fiom year to
year in infected seed of pea, lentil and broad bean (Hampton 1984). Resistant varieties
and use of Wus-6ee seed are the main control strategies for this virus. Control of aphid
populations has not proven a reliable control method (Zimmer & Lamb 1993).
2.42 Pea Enation Mosaic Virus (PEMV)
This is considered to be the most important virus disease of peas in the United
States, and is widespread in most U.S. pea producing regions (Hampton 1984).
Symptoms:
If infection fiom PEMV occurs early, plant distortion and death may occur.
Symptoms of later infections include chlorotic fiecks, stunting, upward rolling of leaves
42
and pod distortion. Foiiage may also develop elongate, chlorotic, tranducent lesions.
Seed size and qudity are ofien reduced. Tissue proiiferation may occur dong leaf veins
and on pods especially on some of the older susceptible varieties (McWhorter & Cook
1958; Hampton 1984).
Disease Cycle:
PEMV overwinters in leguminous crops or weeds. Alfalfa (Medicago sativa) was
until recently considered to be the primary reservoir of PEMV, but now has been shown
to be a non-host of this virus (Larsen et al. 1 996). Aphids, primarily the pea aphid
(Acythosiphon pisum L.), transmit the virus in a persistent mariner (feeding on infected
plants, incubation for 8- 1 2 hours and transmission on subsequent feedings) to host crops.
Transmissibility by the aphids is usuaily Iost after several feeding episodes. The virus c m
also be transmitted by mechanical means (Hagedorn et al. 1964). Cicer. Lathyrus. Lens,
Lupinus. MeIihu, Phaseoh, Pisum, TnfoZiurn and Vicia are known leguminous hosts
of PEMV (Hampton 1984).
Prevention and Control:
The primary method of control of PEMV is the use of tolerant varieties.
2.43 Bean (Pea) Leaf Roll Virus (BLRV)
Symptorns:
Severe stunting and death are symptoms of eariy infection by BLRV. Later
infections can cause chlorosis of only the plant tips or of the whole plants. Stuntuig may
43
also occur with Iate infections. Tolerant varieties may only show slight stunting or may
be symptomless. BLRV is often more severe if other v-es are already ùifecting the
plant.
Disease Cycle:
BLRV is a rnember of the luteovinis group and is transmissible by aphid vectors
in a persistent manner. Seed-borne transmission is not lmown to occur. BLRV
overwinters mainly in perenniai legumes such as alfdfa and clovers and is spread to pea
plants by migrating aphids. The pea aphid (Acyrthosiphon pisum L.) is the most common
vector for this virus. Once infected, the pea aphid can remain viniliferous for the
remainder of its He. After peas are harvested, aphids migrate back to nearby perennial
legumes to overwinter (Hampton 1984).
Prevention and Control:
Use of tolerant vmieties is the best control method presently available. Reducing
aphid populations has provided some control but is not always successful.
2.44 Pea Streak Virus @'SV)
Symptoms:
Sudden death of pea plants results fkom an eariy infection by PSV. Brown or
purple stem and leafspots or streaks are characteristic of a late infection. M y pod
symptoms may occw if inféction occurs drrruig the early bloom stage. Pods dwelop
necrotic spots or sunken pock marks (Hampton 1984).
Disease Cycle:
PSV is a memeber of the carlavirus group that is transmitted in a stylet-borne
rnanner by aphids, rnainly the pea aphid (Acyrthosiphon pimm L.). Ovemintering occurs
primarily in alfalfa, and PSV is transmitted when pea aphids migrate to pea crops in the
s p ~ g and summer. Persisteme of this vims in pea aphids is quite short and is often
dissipated after only four or five feedings. This disease is most severe when pea fields are
planted next to dfalfa fields (Hampton & Weber 1983a; Hampton 1984).
Brevention and ControI:
Use of tolerant varieties will help to keep this disease under control. Control of
aphids in pea fields and nearby alfdfa fields wili reduce the incidence of this virus but
not completely eliminate it. Planting pea fields away from alfalfa fields d l also help tu
reduce the incidence of PSV.
2.45 Pea Stunt
Pea shint is caused by the red clover vein mosaic virus (RCVMV). Common in
red clover and other clovers, this vins together with the pea enation mosaic virus can
cause a desiructive viral complex in pea fields (Hagedorn & Hanson 195 1; Hampton
1984).
Symptoms:
Susceptiile cultivars are often killed before tbey bloom if infected by R C W
early. Snuiting and terminai rosetting are symptoms of later infections. Vein chlorosis or
necrosis may or may not occur. Low temperatures often have a masking eEect on
symptoms of pea shint (Hagedorn & Walker 1949; Hampton 1984).
Disease Cycle:
RCVMV is a member of the carlovirus group which is transmitted in a stylet-
borne manner by aphids, primarily the pea aphid (Acyrthosiphon pisum L). The virus
overwinters rnainly in perennial red clover fields and is çpread when aphids migrate in
the spring and early summer. When peas are harvested, aphids migrate back to clover
fields compieting the disease cycle (Hampton & Weber 1983b; Hampton 1984).
Prevention and Controi:
The use of tolerant varieties is the best means of controlling this viral disease.
Chapter 3 - Field Survey for Pea Diseases
This s w e y was carried out during the 1997 growing season. The puipose of the
survey was to assess root rot incidence and severity and to identie root and foliar
diseases of field peas occumng in the Peace River region of Alberta.
3.1 Materials and Methods
3.1 1 Selection of fields to be suweyed
The sampling region was divided into 6 geographic areas which were separated
by ratura1 existing features such as rivers or hills, and agronomie retail centres where
farmers normally sold their crops and purchased seed, fertilizers, machinery parts, etc.
(Figure 3). A proportional weighting was given to the number of growers in each
geographic area using names fiom a mailing list supplied by the Alberta Pulse Growers
Commission. Area 1 had 12 fields surveyed and encompassed the Municipal District
(M.D.) of Northem Lights and the M.D. of MacKenzie. Area 2 had the highest number of
growers with 33 fields surveyed and included the M.D. of Peace, M.D. of Fairview, and
M.D. of Clear Hills. Area 3 had 1 1 fields surveyed and comprised the M.D. of Spirit
River, M.D. of Birch Hills and M.D. of Saddle Hills. Area 4 had the second highest
number of growers with 29 fields surveyed and included the County of Grande Prairie
and the western part of the M.D. of Greenview. Area 5 had eight fields surveyed and
encompassed the M.D. of S m o b River and M.D. of East Peace. Area 6 had 10 fields
surveyed and included the M.D. of Big Lakes and the eastern part of the M.D. of
Greenview.
! .. . . r..
I
i I I I
Figure
1
---_ ---
. - 7
- . .
48
Growers in each area were selected without bias kom the list, contacted in early
June by phone and asked if they had seeded peas in the spring of 1997. If they had, the
objectives of the disease survey were explained to them and they were asked if they
wanted to participate in the survey. Growers could include as many pea fields as they
wanted. A total of 103 fields were sampled from 74 growers. The survey was conducted
f b m mid-June to late August.
3.12 Sampling Method
Sampling of pea plants was done without bias and a minimum of 50 plants were
collected from each field. The plants were collected by first walking 50 paces into a field.
digging up 10 plants in a row, and continuing on for 5 locations along the arms of a' W'
pattern (Figure 4).
Figure 4: Field sampling method used to obtain pea plants for this study
Enter field here Exit here
50 poccs
5
53 puer
49
Sampling was conducted at three different times during the growing season - at
the seedluig stage (late June to early July), during flowering (rnid-July to late Juiy) and
afier pod set (mid to iate August) in each field to detect both early and Iate diseases and
to observe the progress of various diseases during the summer. In total, fiom the 3
sarnpling &es, approximately 15,450 plants were to be collected (103 fields * 10 plants
* 5 locations * 3 dates = 15,450 plants).
Growers were contacted again during the summer and requested to complete a
questionnaire on crop rotation, cultural practices and pest control used in each field
surveyed. A copy of the questionnaire is included in Appendix 1. This was dune to see if
there was a correlation between management practices and root rot incidence and
severity .
Daily growing season precipitation and temperatures were collected fiom various
weather stations amund the Peace River region. Figures 5 - 9 show growing season
precipitation and Figures 10 - 14 show growing season mean temperatures for selected
locations in the region. T m - y e a r averages are included as a cornparison.
Figure 5: 1997 Growing Season Precipitation -Grande Prairie Figuru 6: 1997 Gmwing Season PmcipitnUon - Falher
June July Month
June July Aug Month
30 yr. avg.
Figure 7: 1997 Growing Season Prccipitation - Peace River Figura 8: 1997 Growing Season Precipitation - High Level
200 - 1 6 0 4 i ;
May June July Aug Month
May June Juiy Aug Month
1997 30 yr. avg. 0 1997 30 yr. avg.
Figure 9: 1997 Growing Seoson Prccipitation - Overall Peace Region
120 - 118.6
May June July Aug Month
Source: Aiberta Agriculture, Food & Rural Development - Conservation & Development Branch
Fbum to: 1997 Growing Sealon Mern Tarnpsrotum - Grande Prairie Figure 11: 1397 Growing Saaron Mean Tempamhim - Folhsr
20 - 20 -
May June July A 4 Month
Figure 12: 1997 Growing season Mean Temperature - Peace River
May June July Month
May June July A W Monîh
1997 30 yr. avg.
Figure 13: 1997 Growing Season Mean Temperature - High Level
20 1
May June July *ug Month
0 1997 30 yr. avg.
Figure 14: 1667 Growing Searon Mewn Tarnpeniure - Overall Peace Rapion
20 -
May June July Aug Month
Source: Alberta Agriculture, Food & Rural Development - Conservation & Development Branch
3.13 Root Rot Pathogen Identification
Sampled plants were brought to the Iaboratory within 48 hr after collection and
stored in a cooler at 5OC for a maximum of 7 days. Roots were washed under tap water
and severity ratings were assigned based on a scale of O to 4, where O = healthy root with
no discolouration, 1 = 1 - 10% root discoIouration, 2 = I l - 25% root discolouration, 3 =
26 - 50% root discolouration, and 4 = 51 - 100% root discoIouration (Howard et al. 1995;
Hwang et al. 1995b; Hwang et al. 1997).
From the 50 plants collected per field, five plants with signs of root discolouration
were then selected without bias for pathogen identification.
Segments of diseased roots were cut, surface-sterilized for 90 seconds in 70%
alcohol and plated onto water agar (WA). Records were made of whether discoIouration
occurred at the top, rniddle or bottom of the main root. Plates were stored at room
temperature (2 1 OC) for 1 to 3 days until rnycelial growth appeared. Mycelia were
transferred to potato dextrose agar (PDA) and stored at room temperature for 10 to 14
days until identification. Identification was done by cornparhg plates to plates with
known pathogen cultures. Known pathogen cultures were obtained from Dr. S.F. Hwang,
plant pathologist with the AIberta Research Council, Vegreville, Alberta No attempt was
made to identifi the pathogens with a microscope and reference key.
3.14 Foiiar Pathogen Identification
While assessing for root rot, records were kept of any spots, lesions or
discolouration present on the foliage, stems, pods or tendrils. These plants were set aside
and five were selected without bias for identification. Sections of diseased areas were
53
surface-stenlized for 30 to 60 seconds in 70 % alcohol and plated ont0 WA. Lacation and
a brief description of spots or lesions was noted. Plates were stored for 1 to 3 days at
room temperature (21°C) until mycelial growth appeared. This was then transferred to
PDA and stored at room temperature for 10 to 14 days until identification. Identification
of foliar pathogens was dso done by comparing plates to plates with known pathogen
cultures.
3.15 Statistical Anal ysis of Suwey Questionnaire
Results from the questionnaire were used to determine if any correlations existed
between agronomie practices used by growers and the incidence and severity of root rot.
Results were statistically analysed using SAS software from the SAS hstitute, Inc.
(1990). Root rot incidence was statistically analysed using a Chi-square test for non-
pararnetric data. The Chi-square test was chosen for analysing incidence for two reasons.
One is that since incidence is a recording of whether root rot is present or absent, the Chi-
square test is ideal for testing for significance. The second reason is that the analysis is
also more accurate when a large number of samples are analysed. This study dealt with
over 15,000 plants that were visually checked for presence or absence of root rot.
For root rot severity, a General Linear ModeIs (GLM) Procedure was used to
check for significant differences. If a significant difference was found, a Student-
Newman-Keuls (SM) test was used to determine Ieast significant differences between
means. Square root transformation of the original root rot severity data was also
computed to improve normality of the raw data and is reported here. Ln dl cases, it
lowered the coefficient of variance and showed similar or greater significance.
3.2 Results
3.21 Incidence and Severity of Root Rot
Pea plants with root rot were found in d of the fields s w e y e d Mean incidence
and seventy of disease increased in al1 areas as the summer progressed. The mean
incidence and s e v e r i ~ of root rot for the three sarnpling times were 88% and 1.2,88%
and 1.5, and 95% and 2.3, respectively (Table 1). The total number of fields vmied £kom
sampling time to sampling time because of worked under crops, road construction and
harvested fields.
Table 1: Incidence and severity of root rot in the Peace River region in 1997
Root rot severity ratings: O = healthy with no root discolouration 1 = 1-10% root discolouration 2 = 1 1-25% mot discolouration 3 = 26-50% root discolouration 4 = 5 1- 100% root discolouration
&ezt
1
2
3
3
5
6 I
Sample 1 Sample 2
n of
fields
12
33
1 1
29
8
10
IO3
X of
fields
13
33
1 1
23
7
IO
96
Sample 3
Severity 0 -4 $of
fields
12
33
1 1
26
7
I O
99
Incidence %
Mean
1.1
1.1
1.0
13
1.0
13
Mean
83
83
89
94
93
95
Range
0.6-1.9
0.4-3.0
0.61.4
0.623
03-12
1.0-1.8
incidence %
Range
55-100
36-100
61-100
62-100
81-100
85-100
12
Mean
78
91
92
95
51
Incidence % Severity 0- 4
88
Range
55-100
56-100
87-100
86-100
13-100
Meai
98
90
100
98
85
99
Mean
12
1.6
1.5
1.8
0.8
1.6
S e v e r i ~ 0 - 4
98 91-100
88
Range
87-100
28-100
100-100
76-100
69-100
94-100
Range
0.7-1.8
0.8-2.9
1.0-2.1
0.84.0
0.2-2.5
1.1-2.2
Mean
2.6
1.8
2.4
2.6
1.9
2.7
95 15
Range
1.8-3.5
0.4-3.8
2.1-2.9
1.44.0
13-33
1.8-3.6
2 -3
3.22 Pathogen Identification
3.221 Root Rot Pathogens
The main root rot pathogen identified in this survey was Fusarium spp. which
occurred in 99 of the 103 fields surveyed. Of the four remaining fields, one had
Rhizoctonia spp., one had Bowîis spp. and the other two were unidentified No other
pathogens were positively identified. Eight other fields also had Rhizoctonia spp. and six
had Bovtis cinerea dong with Fusankm spp.
3.222 Foliar Pathogens
Only 1 6 of the 1 03 fields in the first sampling tirne showed any foliar symptoms.
Seventy of the 96 fields in the second sampling time had foliar symptoms and dl the
fields in the third sampling time had some foliar symptoms. Foliar pathogens identified
were Ascochyta spp ., Srlerotinin sclerotiorum, Fusarium spp ., and Botrytis cinerea.
The two main diseases identified fiom the symptoms observed on plants were
mycosphaerella blight and sclerotinia rot. Symptoms of mycosphaerella blight were
present on plants fiom al1 the fields by the third sampling t h e . Symptoms of sclerotinia
rot were observed on plants nom 54 of the 99 fields in the third sampling time.
Five samples of isolated Fusariurn spp. were selected without bias and sent to Dr.
Keith Seifert, (Eastern CereaI and Oilseed Research Centre of Agriculture and Agri-Food
Canada, Ottawa, Ont.) for identification to species. Three were recovered from root
lesions and two fiom f o l k lesions. Two of the root isolates and both of the f o l k isolates
were identified as F. avenaceum (Fr.) Sacc. The other root isolate was identified as F.
sambucinum Fueckel.
3.23 Suwey Questionnaire Results
1. How many acres of field peas are you growing in 1997?
Acreage of field peas grown varied widely across the region with the mean acreage at
93.0 hectares (standard deviation of 67.1 hectares). The overall range was fiom 4.9
hectares to 323.8 hectares. Table 2 gives the mean acreage, standard deviation and range
for each area surveyed. The 74 growers surveyed grew 6,922 hectares of field peas in
1997. Based on the 1997 acreage of 28,329 hectares of field peas in the Peace River
region (Boje W. - Personal Communication 1998), this represents approximately 24
percent of the pea acreage in the region.
Table 2: Mean acreage, standard deviation and range of fields suweyed
Area
1 l
2
3
4
5
6
Overall
Number of fields
12
Number of growers
11
20
IO
20
4
9
74
Range (hectares)
44.5 - 234.7
Mean Acreage ' (hectares)
107.1
8.1 - 242.8
20.2 - 129.5
16.2 - 234.7
80.9 - 323.8
4.9 - 229.9
4.9 - 323.8
Standard Deviation (hectares)
58-4
57.8
40.3
64.6
109.8
67.7
67.1
33
11
29
8
10
1 03
77.8
68.2
105.8
194.3
63.6
93.0
2. How many years have you been growiing field peas?
1st time 1 year 0 2 years 3 or more years
Seventy-two of the 74 growers (97.3 %) sweyed had 3 or more years experience in
growing field peas. Of the remaining two, one was a first time pea grower (1.35 %) and
the other had 2 years experience (1 -35 %).
3. What has been the cropping history in this field?
The crop immediately preceeding peas was wheat in 47.6 % of pea fields surveyed in
1997, followed by barley (2 1.4 %) and canola (20.4%). Canola was the most common
crop grown in these fields in 1995 and 1994 (43 -7 % and 29.7% of fields surveyed,
respectively). Wheat was the most common crop grown in 1993 and 1992, in 34.5% and
38.5% of fields surveyed, respectively. Only two of the 103 fields surveyed were seeded
ont0 pea stubble. Table 3 shows the number of fields of each crop type over the 5 years
before the swey. Only 1 996 and 1 995 have cropping history for al1 fields in the survey.
Some growers had not kept records and could not remember back 5 years. Others had not
f m e d the land for the full 5 years and did not h o w previous cropping history.
4. Have you had any diseases in your peas in previous years? If yes, did you have it
identified?
Most growers (72%) had not noticed any disease in their pea crops in previous years. Of
the 28% of growers that had noticed disease in their pea fields in previous years, 15% did
not know what disease it was and did not have it identified, 6% reported ascochyta blight,
4% reported root rot, 2% reported sclerotinia rot, and 1% reported downy mildew.
Table 3: Cropping history of pea fields surveyed
5. Did you use certified or better seed? If yes, what variety of peas are seeded in this
field?
Certified or better seed was the most prevalent seed source with 55.9% of growers having
used it. The remahhg 44.1% of growers used their own seed or purchased it fiom a
neighbour (common seed). Of the certified or better seed used, Carneval was the most
common variety, with 18.6% of growers planting it, followed by Espace with 1 1.8%,
Majoret with 8.8%, and Carrera with 4.9%. 0 t h varieties used were Caderos, Highlight,
Mustang, Orb, Eiffel, Danto, 1141, Ascona, Profi, and Grande.
%
43.7
0.0
26-2
5.8
1.9
0.0
2.9
2.9
10.7
0.0
2.9
2.9
fieIds
1995
45
O
27
6
2
O
3
3
11
O
3
3
1 03
Crop Number of
1996 %
canola
flax
wheat
barley
oats
canqseed
creeping red fescue
bromegrass
Peas
alfa1 fa
hay/pasture
summerfaliow
# of fields
1994
30
O
28
12
3
O
2
3
IO
O
7
6
101
21
O
49
22
2
1
% --------- 29.7
O .O
27.7
11.9
3 .O
0.0
2.0
3 .O
9.9
0.0
6.9
5.9
20.4
O
47.6
2 1.4
1.9
1 .O
1993
16
1
29
7
2
O
5
1
13
2
5
3
84
L
2
2
O
1
2
1 03
1 .O
1.9
1.9
O
1 .O
1.9
%
13.8
0.0
38.5
16.9
0.0
0.0
9.2
1 -5
0.2
3.1
9.2
1.5
t
%
19.0
1 -2
34.5
8.3
2.4
0.0
6.0
1 -2
15.5
2.4
6.0
3.6
1992
9
O
25
11
O
O
6
1
4
2
6
1
65
59
6. Did you use seed treatment? If yes, please iodicate which one you used.
Thiram 7 5 W D Captan FL 0 Apron FL Other
Most growers (69.7%) did not use seed treatment. Of the 30.3% who used it, 17.1% used
thiram, 6.6% used metalaxyl, 5.3% used a combination of diiram and metalaxyl, and
1.3% indicated they had used another type of seed treatment. None of the growers used
captan.
7. Did you use inoculant? If yes, what type was used?
peat powder liquid granular
Inoculant was used in 102 of the 103 fields surveyed. Granular soi1 inoculant was the
most common inoculant used by 48.7% of growers. Peat powder seed inoculant was next
at 40.8% and liquid seed inoculant was used by only 10.5% of growers.
8. Did you apply any fertilizer? If yes, please indicate type, rate and method of
application.
Most growers (74.7%) appiied fertilizer with their pea seed. The most common fertilizer
used was 12-5 1-0, with 46.7% of growers having used it. Other fenilizers used were
various blends of nitrogen, phosphoms, potassium and sulphur. Five fields had anhydrous
ammonia (82-0-0) applied prior to seeding of the peas. Rates varied as much as did types
of fertilizer blends.
The most common method of application, used by 67.9% of growers, was with the seed,
followed by banding at 30.4% and broadcasting at 1.7%.
9. Were your peas zero-tilled?
yes If yes, how many years has this field been zero-tilled?
1st t h e U 1 year 2 years 3 or more years
0 no If no, how many tillage passes were made prior to seeding?
one tillage p a s 0 two tiiiage passes three or more Nage passes
Approximately one-third (30.3%) of the fields surveyed were zero-tilled. Forty percent of
fields were minimum-tilled having received only one tillage pass prior to seeding of the
peas. The remaining 29.7% of fields were conventionally tilled receiving two or more
passes before seeding.
Of the zero-tilled fields, 37.9% were zero-tilled for the first tirne, 10.4% had been zero-
tilled the previous year, 27.6% had been zero-tilled for the last
been zer&lled for at Ieast three years.
10. Did you use a land rouer after seeding? If yes, when did
immediately after seecàing 0 before emergence of peas 0
two years, and 24.1 % had
you roii the land?
after emergence of peas
Most growers (82.9%) did not roll their land after seeding peas. Of the 17.1% of growers
who rolled their fields, 6 1.5% rolled after emergence of the peas, 23.1 % rolled before
emergence of the peas, and 15.4% rolled immediately after seeding.
11. Have you used any herbicides on this pea field? If yes, please indicate type and
rate used.
Ten growers (13.5%) did not use any herbicides on at l e s t one of their fields. Of the
remahing 64 growers (86.5%) that used herbicides, the most common herbicides were a
combination of the broadleaf herbicide imazethapyr (Pursuit) and the gras herbicide
sethoxydim Poast). This mixture was applied to 42.7% of the fields in the survey. The
next most commonly used herbicide was imazamox and imazethapyr (Odyssey) sprayed
on 12.6% of fields, followed by imazethapyr alone applied to 9.7% of fields. Imazethapyr
was a very commonly used herbicide on fields in this survey. It was used alone or in
combination with other herbicides on 56.3% of fields surveyed. Other herbicides used
were Select (clethodim), Sencor (metnbuzin), Venture (fluazifop-p-rnethyl), MCPA
amine, MCPA Na, Assure (quizalofop-ethyl), Hoegrass (diclofop-metyl), Troporox Plus
(MCPB + MCPA) and Roundup (glyphosate).
12. What herbicides were used in this field in the last two years? (1996 and 1995)
Thirty difTerent herbicides were used in the 2 years pior to this survey. Of these, only
four, Lontrel (clopyralid), Prevail (tralkoxydim + clopylid + MCPA ester), Muster
(etharnetsulfixon-methyl) and Ally (metsulfùron methyl) could pose residual problerns
that might affect peas. In 1996, 11 fields had one or more of these herbicides applied to
them. In 1995,2 1 fields had one or more of these herbicides applied to them.
3.24 Results of Statisticd Analysis
3.241 Differences Between Areas
The first test was for differences in root rot incidence and severity between areas.
Areas were divided as follows: (Figure 3)
Area 1 : Manning/Forî Vendion Area 4: DeboltGrande Prairie Area 2: BemydFairview Area 5: FalherPeace River Area 3 : Spirit RiverEaglesham Area 6: Valleyview/High Prairie
62
Incidence
In the f h t sampling time, there was an association between root rot incidence and
the area in which peas were grown, ie., incidence of root rot varied between different
areas. The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at 94.9%
and the lowest was in area 2 (Berwyn/Fairview) at 82.3%. In the second sampling tirne,
an association between root rot incidence and the area in which the peas were grown was
also found. The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at
97.9% and the lowest was in area 5 (FalherReace River) at 51.4%. In the third sampIing
tirne, there was also an association between root rot incidence and area. The highest
incidence of root rot was in area 3 (Spirit RiverEaglesharn) at 100% and the lowest was
in area 5 (Falhermeace River) at 85.2%. When root rot incidence was averaged over the
three sampling times, an association was also found between root rot incidence and area.
The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at 97.4% and
the Iowest was in area 5 (Falher/Peace River) at 76.9%. The number, the percent and the
Chi-square probability for area variation by root rot incidence can be found in Table 4.1.
Severity
No differences were detected in root rot severity between the six areas in the fnst
sampling tirne. h the second sampling time, root rot severity in area 5 (FalherPeace
River) was significantly lower than in areas 1,2,3,4 and 6. Ln the third sampling tirne,
root rot severity in area 5 (FalherfPeace River) was significantly less than in areas 3 , 4
and 6 but not area 2 (Berwyn/FaWiew). Root rot severity in area 2 (Berwyn/r;airview)
was significantly lower than in areas 3 , 4 and 6 but not area 5 (FalherPeace River).
63
When averaged across the 3 sampling M e s , root rot severity in area 5 (FalherPeace
River) was significantly less fhan in areas 1,2,3,4 and 6. Significant differences in
overall root rot severity between different areas are reported in Table 5.
3.242 Cropping History
Previous cropping history is known to 8 e c t the incidence and severity of root rot
in peas (Kraft 1984). h this analysis, crops were combined into three groups for testing:
1. Cereals: wheat, barley, oats, canaryseed, bromegrass and creeping red fescue (for seed) 2. Oilseeds: canola 3. Legumes: peas and hay/pasture
Summerfdlow fields were omitted since no crop was grown on these fields in the
previous year. Only 1996 and 1995 cropping hîstory was used because the cropping
history of dl the fields was not available in the other years.
Incidence
In d l three sampling times and when averaged overall, there was an association
between root rot incidence in the 1997 pea crop and the type of crop grown in both 1996
and 1995. Fields that had canola in 1996 had the highest incidence of root rot (9 1.7%)
while fields that had cereals had the lowest (90.1%). Fields that had legumes in them in
1995 had the highest incidence of root rot (95.7%) while fields that had oilseeds had the
lowest (89.1 %). The number, the percent and the Chi-square probability for cropping
history variation by root rot incidence can be found in Table 4.1.
Severity
There was no significant difference in root rot severity of the 1997 pea crop between the
three groups of crops grown in 1996 in al1 three sampling times and when averaged
64
overall. For 1995 crops, the fmt and second sarnpling times showed no difference in root
rot severity between the three groups of crops, however, in the third sampling tirne, root
rot severity was significantly higher if legumes had been grown than if oilseeds had been
grown. There were no significant differences between legumes and cereals.
Overall, there was a significant difference in root rot severïty. If legumes were grown in
the field in 1995, root rot severity was çignificantly higher in the 1997 pea crop than if
cereals or oilseeds were grown. Significant differences in overdl root rot severity
between previous crops are reported in Table 5.
3.243 Disease in Previous Pea Crops
Incidence
Only the second sampling time showed an association between root rot incidence
and growers who had noticed disease in prior pea crops. Growers who had not noticed
disease in their pea fields in prior years had a higher level of root rot incidence (90.2%)
than growers who had noticed disease in their pea fields in previous years (82.9%).
Severity
There was no signifïcant difference in root rot severity between growers who had
noticed disease in their previous pea crops and growers who had not.
3.244 Seed Source
If the growers used their own seed or bought uncertified seed fkom neighbours,
this was known as common seed. If growers purchased certified or better seed, then a
varietal name could be attached to the pea seed. Certified seed was used by 55.9% of
growers and common seed was used 44.1 %.
65
Incidence
There was an association between root rot incidence and seed source in al1 three
sampling times and when averaged overall ie., there were significant differences in root
rot incidence between certified and common seed. Growers who used certified seed had a
higher incidence of root rot (92.8%) than growers who used comrnon seed (87.4%). The
number, the percent and the Chi-square probability for cropping history variation by root
rot incidence can be found in Table 4.1.
Severiiy
In the f ~ s t and thïrd sampling times, there was no difference in root rot severity
between common and certified seed.
In the second sampling time and when averaged overall, however, root rot severity was
significantly higher when certified seed was used rather than comrnon seed. Significant
differences in overall root rot severity between seed sources are reported in Table 5.
3.245 Seed Treatrnent
Differences between metalaxyl, thiram and no seed treatment were tested in this
analysis. No growers used captan and only one grower had used another unknown seed
treatment.
Incidence
There was an association between root rot incidence and use of seed treatment in
the second and third sampling times and when averaged overall but not in the e s t
sampling time. Metalaxyl treated seed had the highest incidence of root rot (93.5%) and
thiram treated seed had the lowest (88.7%). The number, the percent and the Chi-square
66
probability for seed treatment variation by root rot incidence can be found in Table 4.1.
Severiîy
The only significant difference to show up in root rot severity was in the second
sampling t h e . Metalaxyl treated seed had significantly higher root rot severity than
thiram treated seed and untreated seed There was no difference between thiram treated
seed and untreated seed. When averaged across al1 three sampling times, no significant
difference in root rot severity was found.
3.246 InocuIant Formuiation
Incidence
There was an association between the three types of inoculants used and
incidence of mot rot in al1 three sarnpling times and when averaged overall. The highest
incidence of root rot was when no inoculant was used (96.7%) and the lowest was when a
peat based seed inoculant was used (88.7%). The number, the percent and the Chi-square
probability for inoculant formulation variation by root rot incidence can be found in
Table 4.1.
Severity
No significant differences were observed between root rot severity and the type of
inoculant used.
3.247 Fexailizer Use
There were 26 different kinds of fertilizer blends used by the various growers. To
simp1iQ the analysis, groupings were done.
The three groups were:
1. Nitrogen + Phosphorus (5 1 fields) 2. Nitrogen +/or Phosphorus +/or f otassium +/or Su1fb.r (27 fields) 3. No fertilizer used (25 fields)
Incidence
There was an association between root rot incidence and fertiiizer use in al1 three
sampling times and when averaged overall. The highest incidence of root rot occurred
when a blend of nitrogen a d o r phosphorus and/or potassium anaor sulfur was used
(92.3%) and the Iowest incidence occurred where no fertilizer was used (87.6%). The
number, the percent and the Chi-square probability for fertilizer variation by root rot
incidence cm be found in Table 4.2.
Severiîy
In the frst and third sampling times, root rot severity was significantly higher
when fertifizer was used than when no fertilizer was used. There were no diflerences
between the iwo types of fertilizer used. In the second sampling time and when averaged
across the three sampling times, there were no sigiificant differences in root rot seventy
between the three groups.
3.248 F e r m e r Application Method
Incidence
There was an association between root rot incidence and fertilizer application
method in al1 three sampling times and when averaged overali. The highest incidence of
root rot occurred when fertilizer was banded (94.5%) and the lowest b e l occurred when
fertilizer was placed with the seed (90.1%). Al1 three methods of application had higher
68
incidence of root rot than when no fertiher was used. The number, the percent and the
Chi-square probability for variation of fedlizer application method by root rot incidence
c m be found in Table 4.2.
Severity
For the f ist sarnpling Mie , root rot severity was significantly higher when
fertilizer was either banded, broadcast or when it was applied with the seed compared to
when it was not used at dl. In the second sampling time, there were no significant
differences between fertilizer that was banded, broadcast or applied with the seed. In the
third sampling time, root rot severity was significantly higher when fertilizer was
broadcast or banded than when it was applied with the seed or not used at dl. When
avemged overall, root rot severity was significantly higher when fertilizer was broadcast
or banded than when it was not used at dl . There was no difference between fertilizer
that was broadcast or banded and fertilizer that was applied with the seed- There was also
no difierence between fertilizer that was applied with the seed and when no fertilizer was
used. Significant differences in overall root rot severity between fertilizer application
methods are reported in Table 5.
3.249 Tiiiage
Incidence
In the first and third sampling tirnes, and when averaged overall, there was an
association between root rot incidence and type of tillage. No association showed up in
the second sampling time. Fields that were minimum-tilled had the highest incidence of
root rot (90.9%) while fields that were zero-tilled had the lowest (80.6%). The number,
69
the percent and the Chi-square probability for tillage variation by mot rot incidence cm
be found in Table 4.2.
Severity
No significant differences in root rot severity and type of tillage showed up in any
of the sampling tirnes or when averaged overall.
3.250 Rolling
Incidence
In al1 three sampling times and when averaged overall, an association between
root rot incidence and rolling was found. Fields that were rolled immediately after
seeding had the highest incidence of root rot (97.4%) while fields that were rolled before
emergence or weren't rolled had the lowest level(89.5%). The nwnber, the percent and
the Chi-square probability for rolling variation by root rot incidence can be found in
Table 4.2.
Severity
No significant differences were observed between root rot severity and rolling.
3.251 Herbicide Use
An andysis of whether herbicide use had an effect on root rot intensity and
severity in the 1 997 pea crop revealed some interesting results.
Incidence
In the second and third sampling times and when averaged overall, an association
between root rot incidence and whether herbicides had been used or not was found. No
association was found in the first sampling tirne. Use of herbicides caused a higher
70
incidence of root rot (9 1.3%) than when no herbicides were used (8 1.9%). The number,
the percent and the Chi-square probability for herbicide use variation by root rot
incidence can be found in Table 4.2.
Severity
The first sampling time revealed no significant differences in root rot severity
whether herbicides were used or not; however, in the second and third sampling times
and when averaged overall, root rot severity was significantly higher when herbicides
were used than when no herbicides were used. Significant differences in root rot severity
between herbicide use and non-use are reported in Table 5.
3.252 Herbicide Residues
Incidence
Only the second sampling time revealed an association between root rot incidence
and use of residual herbicides in 1996 that could damage peas. Fields that had been
sprayed with a residual herbicide treatment in 1996 had a higher incidence of root rot
(94.1%) in 1997 than fields that had not been sprayed with a residual herbicide (87.8%).
Severity
No significant differences were fomd between root rot seventy and use of
residud herbicides.
TabIe 4.1: Num ,er, percent and Chi-square probabiiities for sources of variation by overall mot rot incidence
- -
~ o o t Rot ~ r z - ~ l Root Rot Absent
Source
Area
96 Crop
Number of YO Plants
43 2.6
200 4.6
Groupings Num ber of Plants
2
1
5
Oilseeds
Cereals
95 Crop
Variety
Seed
Treaîment
Inoculant
--
Cereals
Oilseeds
Certified - ppp
Common
Thiram
Liquid
Peat
Table 4.2: Number, percent and Chi-square psobabilities for sources of
Source
Fertiiïuer
Application
Method
Tiage
Rolling
97 Herbicides
variation by overd root rot incidence
I
After 479 97.4 13 seeding
r i
Groupings
NPKS
NP
None
Banded
Broadcast
With seed
None
After 1 1892 / 95.2 1 emergence
- -
Before 1 668 1 89.5 1 78 1 10.5 emergence 1 I I I
Root Rot Present RootRot Absent
Number of Plants
3934
7350
3544
3166
305
7813
3544
Number of Plants
328
734
502
185
20
857
502
YO
9 2 3
90.9
87.6
94.5
93.9
90.1
87.6
YO
7.7
9.1
12.4
5 5
6.2
9.9
12.4
None
Herbicides
None
11789
13677
1151
10.5
8.7
18.1
I
89.5
91.3
81.9
1378
13 10
254
Table 5: Overall mean severity, coefficient of variance, F-value and Pr > F for root rot severity by source of variance
Groupings Mean Coefficient Severity of
Variance
Source
Area
95 Crop
Legumes
Variety Common -
Certified
Application
Method With seed
97 Herbicide No herbicide
Herbicide
a,b Means followed by the same letter are not statistically different at the P = 0.05 level of probability
3.3 Discussion
The mean root rot incidence and seventy for the îhree sampling times ie., 88%
and 1 .2,8 8% and 1.5, and 95% and 2.3, respectively, are fairly high given what has been
reported previously in the Peace River region and in other parts of Alberta. Harrison and
Laflamme (1 W6), in a root rot survey of Peace River region pea fields in 1995, found a
mean disease incidence and severity of 64% and 1.2, respectively. Hwang et al. (1997) in
a root rot survey of north-centrai Alberta pea fields found a mean root rot incidence and
severity of 34% and 0.5, respectively. Howard er al. (1995) in a root rot survey of pea
fields in southern Alberta reported a disease incidence and severity of 6 1 % and 1.2,
respectively.
One reason for the differences may be the t h e the s w e y s were conducted.
Hwang et al. (1997) and Howard et al. (1995) conducted their surveys only once in June,
which is quite early in the season. The root rot figures of 88% and 1.2 in the first
sampling time are very similar to what is reported in the rest of the province. The
incidence and severity in this s w e y were found to hcrease as the season progressed.
Consequentiy, it can be assumed that the later in the season that a root rot assessrnent is
done the higher will be the incidence and severity.
A second reason for these differences in results codd be due to the higher than
normal precipitation recorded in 1997. As can be seen in Figures 5 - 9, most of the Peace
region expenenced precipitation well above the 30-year average. This combined with
cool temperatures in May (Figure 14) (1997 average of 9A°C vs. 30-year average of
75
10.0 OC) which reduced evapotranspiration may have provided an environment which
favoured root rot pathogens.
There was some concern about the use of plastic bags to store plants and the
amount of time needed to transport the samples back to the lab. Distance was the main
factor for the amount of time it took to get the samples back to the lab. For example, the
distance fkom the lab in Fairview where the samples were stored to Fort VermiIion is 4 15
kilometres. It was impossible to drive to this area, collect pea plants fiom the 12 fields in
this area and r e m to FaWiew in one 24-hour period. Most of the samples were in the
cooler on the same day they were collected and none of the samples was more than 48
hours old before being put into the cooler. To decrease the rïsk that samples were not
being colonized by secondas. pathogens, plants were visually checked for any signs of
secondary colonization. Only one set of samples fkom the third sampling time in area 6
(ValIeyview/High Prairie) showed signs of secondary colonization. These plants had
been collected on a showery day and were quite wet when put into the bags. They were
discarded and fiesh samples were collected to replace them.
The identification of Fusan'um spp. as the main soi1 root rot pathogen is
supported by similar findings in other parts of Alberta (Sumar & Howard 1979; Hwang
& Chang 1989; Howard et al. 1995). F. avenaceum was identified as the species isolated
on the plates. This species is very common in Alberta soils and is often isolated fkom the
roots of peas, lentils, wheat, barley, canola and many other crops grown in Alberta. It is
also known to be very fast growing on PDA culture (Howard R. J. - Persona1
Communication 1998; Hwang S.F. Personal Communication 1998). This could be the
76
reason why it was the Furarium species to be identified as the samples sent for
identification were fiom plates that had been in culture for one to two months. F.
uvenacmm may have grown over everything eIse in the plate.
No Pythium spp. were identified in this survey. This rnay be due to the time of
year when the survey was conducted. Fythium spp. are known to occur early in the
season when soils are relatively cool and moist @ m a n 1984a; Hwang et al. 1997). As
soi1 warms up, Fusarhm spp. become more common and may overwhelm the Pythium
spp. Had the first sampling tune occurred soon after emergence of the peas, Pythiurn spp.
might have been isolated.
Foliar pathogens proved to be very difficult to identiw fiom culîure. Only four
plates were positively identified as having Ascochyta spp. even though visual symptoms
of the disease were present on most of the plants. It could be that the pathogens were
inactive because of dry conditions at the time of collection. Another reason may have
been the use of PDA to isolate the pathogens. The use of a selective medium for
Ascochyta spp. might have been a more usefbl method to isolate diis species in culture.
Syrnptoms of sclerotinia rot were also present on numerous plants fiom the third
sampling time but only nine plates confhed the presence of Sclerotinia sclerotiorurn.
Again, it could be that the pathogens on many of the plants were inactive because of dry
conditions at the time of collection.
3.31 Effect of Agronomic Practices on Root Rot Incidence and Severity
3 3 11 Differences Between Areas
Because of the size of the Peace River region, it can be expected that some
77
differences in moisture, soil types and soil pH will be present between areas. This could
account for some of the differences seen in root rot incidence and seventy between areas.
When averaged across al1 three sampling times, root rot incidence was highest in area 6
(Valleyviewfigh Prairie) and lowest in area 5 (Falher/ Peace River). Root rot severity
was significantly lower in area 5 (FalherReace River) than in al1 the other areas.
Area 5 had the Iowest fevel of both incidence and severity. This is an area that has not
followed the trend of the rest of the Peace River region in increasing pea acreage.
Growers in area 5 are wary of growing peas because of poor drainage due to heavy clay
soils and a generally fi at topography. Crops often suffer from excess moisture especially
after a heavy summer rainstorm. The infrequency of peas in cropping rotations in this area
may be one reason why root rot incidence and seventy were lower in this area than in
O ther areas.
It is difficult to explain why root rot incidence and severity in area 5 declined from the
first sampling time to the second sampling time and then went back up again in the third
sampling time. Figures 15 and 16 show this trend. As c m be seen from Table 1, trends for
root rot incidence and severity in areas 2, 3 , 4 and 6 were to increase with each sampling
time. Area 1 had a slight decrease in incidence in the second sampling time but had a high
incidence by the third sampling time. When viewed as an overall region, this same trend
continued. The drop in area 5 of both root rot incidence and severity in the second
sampling time and the lower incidence level in the third sampling time than in the f i t
sampiing time remains an anomaly that is dificult to explain given the nature of this
projet.
1 2 3 Sarnpling Time
Figure 15: Root rot incidence in Area 5 - FalherPeace River
1 2 3 Sarnpling Tirne
Figure 16: Root rot seventy in Area 5 - FalherPeace River
79
3.312 Cropping History
Peas grown in 1997 on fields that had oilseeds planted in 1996 had slightly higher
incidence of root rot (91.7%) then legurnes (90.4%) and cereais (90.1%). Peas grown in
1997 in fields that had legumes in them in 1995 also had higher incidence of root rot
(95.7%) then cereals (90.5%) and oilseeds (88.1 %). The type of crop g r o m in 1 996 did
not have an effect on root rot severity on peas grown in 1997; however the type of crop
grown in 1995 àid have a significant effect. Legumes in the rotation are known to be a
cause of higher levels of root rot (Salt & Delaney 1984; Tu & Findlay 1986) and a 4 to 5-
year crop rotation between legume crops should be practised to ailow pathogen levels to
decrease to where they are no longer causing economic damage (Kraft 1984). Results are
not as clear for peas grown on fields that had legumes in them in 1996. The incidence
levels were much closer together and significance was not as hi& (0.019) as with fields
that had legumes in 1995 (0.00 1). Severity showed no significant differences for fields
that had legumes in 1996. This is interesting since one would expect the most recent crop
to have the largest effect on the current crop. However, when the data are examined more
closely, there were only three fields of legumes in 1996 (two pea fields and one hay
field). ïhis represents oniy 3 percent of the fields in the survey and may not have been
enough to provide a significant difference in root rot severiq.
The situation is different for crops grown in 1995. The larger number of legume
fields (1 1 pea fields and 3 hay/pasture fields) represents 14 % of fields and this time
significant differences showed up. If legumes (peas, hay/pashire) were in the field in
1995, then root rot severity was significantly higher in the 1997 pea crop.
80
3.313 Seed Source
The fact that use of certified or better pea seed resuited in higher incidence and
severity of root rot than use of common seed came as a bit of a surprise. Since none of
the root rot pathogens are seed-borne, one would assume that seed source should not be a
cause of higher root rot- One possible cause of this could be damage occurring to pea
seed due to the greater amount of handling that occurs with certified seed as cornpared to
cornmon seed. With certified seed, seed is hanrested, cleaned, brought back to a bin on
the farm then reloaded to go to the retailer, then sold and brought to the buyer's bin. It is
then reloaded to go into the seeder. Common seed is harvested, cleaned and then put in a
bin until it is needed for seeding. Unless the certified seed is handled very gently with
belt augers (versus screw augers), the extra handikg may result in more cracks
developing on the seed coat. This could result in more exudates leaking fiom the seed
whick wouId attract root rot pathogens and increase both incidence and severity of root
rot (Hamian 1984a; Hwang et al. 1997). Given the nature of this project, the effects of
seed source on root rot cannot be adequately explained.
3.314 Seed Treatment
Metalaxyl treated seed had the highest level of root rot incidence when averaged
across the three sampling times. It also had a significantly higher level of root rot severity
than other treatments in the second sampling tirne. The reason for this may be that since
the main root rot organism identified in this survey was Furankm spp., and metalaxyl is
specific for control of Pythium spp. (Anonymous 1998a), Fusarium spp. were still able to
attack the pea plants resulting in higher incidence of root rot. This does not, however,
81
explain why metdaxyl treated seed had higher incidence of root rot dian untreated seed.
More research would be needed to further expiain this association.
3.315 Inoculant FormuIation
Use of peat based seed inoculant resulted in the Iowest incidence of root rot
(88.7%), followed by granular soi1 inoculant (9 1.4%) and liquid seed inoculant (92.4%).
These differences cannot be adequately explained given the nature of this project. Use of
inoculant Iowered the incidence of root rot in this study. Plants taken from fields that had
inoculant applied had lower incidence of root rot than plants from fieIds where no
inoculant was used (96.7%). However, only one field out of the 103 fields in the survey
had no inoculant applied. Further research wouId be needed to c o n f m this association.
3.316 FertiIizer Use
In this study, use of fertilizer resulted in higher levels of root rot incidence than
when no fertilizer was used. Use of a blend of one or more nutrients had the highest level
of root rot (92.3%), followed by nitrogen/phosphorus (90.9%) and then no fertilizer
(87.6%) which had the lowest level of root rot incidence. Fertilizer use also resulted in
higher levels of root rot severity in the first and third sampling times.
A reason for this may be that use of fertilizer usually results in increased root and
plant growth. This increased growth also results in higher levels of root exudates in the
rhizosphere around the roots which may attract more root pathogens to the roots and lead
to higher Ievels of root rot.
Tu and Findlay (1986) mention in their study that use of high levels of fertilizers often
leads to increased levels of root rot. Srihuttagum and Sivasithamparam (1991) in their
82
study on the effects of fertilizers on root rot in peas, found that al1 treaûnents containing
fertilizers, with die exception of phosphorus/potassium, nitrogen or nitrogen/phosphorus
resulted in higher levels of root rot severity than the no fertilizer treatment. They also
mention that root rot caused by some pathogens such as Fusariurn oxysponun and
Rhizoctonia sulani are reduced with addition of certain fertilizers while other pathogens
such as Pythium vexans appear to increase in root rot severity with the addition of sorne
fertilizers. In their snidy, when dl three pathogens were combined, fertilizer failed to
reduce severity. Since a cornplex of pathogens including Fusarium spp., Rhizoctonia
solani and Pythium spp. are thought to cause root rot in Alberta (Swanson et al. 1984;
Hwang & Chang 1989; Hwang et al. 1995b), the addition of fertilizer may actually be
increasing both the incidence and severity of root rot.
3.317 Fertiiizer Application Method
Banding of fertilizer resulted in the highest incidence of root rot (94.5%) and
placing the fertilizer with the seed resulted in the Iowest incidence (90.1 %). There were
no significant diEerences in severity of root rot between banding, broadcasting or seed
placed fertilizer. The effects of fertilizer placement on root rot may be related very closely
with the use of fertilizer. Regardless of whether the fertilizer was banded, broadcast or
seed placed, root rot incidence and seventy were always higher than when no fertilizer
was used (87.6%). Given the nature of this study, the difference between application
methods cannot be adequately explained.
3.318 Tillage
The lower level of root rot incidence reported here under zero-tillage as compareci
83
to conventional tillage is contrary to other research. There were no differences in root rot
incidence of peas at matunty between zero, minimum and conventional tillage treatments
in some research done in Saskatchewan (Bailey et al. 1992; Bailey et al. 1997). The
moisture difference between the Peace River region of Alberta and central Saskatchewan
where previous studies were done might be one possible explmation for the differences
in results.
This study showed no diflerence in root rot severity between tillage treatments. This
agrees with research done by Gossan et al. (1996), who found no consistent effects of
root rot severity in peas over the 4 years of their study. Turkington et al. (1997), however,
in a 4-year study found that average root rot severity was significantly higher under
conventional tillage cornpared to zero-tillage.
3.319 Rolling
The effects of rolling of a field on root rot incidence and severity in peas has not
previously been reported in the literature. Incidence may be affected more than severity as
no significant differences in severity were seen. The higher incidence of root rot reported
on peas rolled imrnediately after seeding could be due to the eKects of soil compaction
from the weight of the land rolIer on soils that have a higher moisture content. Tu (1987a)
States that peas grown in compacted soils have increased incidence and severity of root
rot. Soi1 moisture is usually higher at seeding time than later on when peas have emerged.
This higher soil moisture at this time could increase the effect of soil compaction.
3,320 Herbicide Use
Herbicide use resuIted in higher levels of both root rot incidence and severity than
84
where no herbicides were used. Use of herbicides always increases the stress put on pea
plants even if there are no visual symptorns. Tu (1992) showed that the use of phenoxy
herbicides such as MCPA and MCPB that cause stress to the peas aIso increased root rot
severity. Generally, however, the amount of stress inflicted by the new chemistry of
herbicides used on peas is minimal and the effect on yield is much less than would occur
if weeds were not controlled. Research on timing of weed removal in peas has s h o w that
10 to 15 bushels per acre is lost for every week that weeds are not removed or controLled
after crop emergence (Harker K-N. - Personal Communication 1998).
Chapter 4 - Pea Disease Management Program
Disease in peas is the result of an interaction between a pathogen, the pea plant
and conditions favourable to development of a disease. A good disease management
program involves manipulating or modifjhg one or more of these parameters to stop the
interaction fi-om occ&g. For example, the use of a resistant variety of peas will not
dlow the interaction between the pathogen and the host pea plant to occur even if
environmental conditions are favourable. This will effectively "control" the disease.
Tbere are many other ways to achieve this "control". This chapter will illusirate the best
ways to manipulate or modi@ the host peas, the pathogens and the environment to
prevent, stop or minimize diseases of pea in the Peace River region of Alberta.
4.1 Cultural Practices
One of the most common ways to control disease is to make changes to the
environment in which the peas are growing. Modi-g cultural practices is the easiest
way to do this.
The type of modifications done will Vary according to the type of farming
operation used by the grower. Conventional tülage and conservation tillage are the two
main types of farming operations in use in the Peace River region of Alberta at present.
Conventional tillage is still the most common type of famiing system in use, with 62 %
of cultivated hectares being farmed using this method. Conservation tillage, however, has
grown in popularity over the last decade and is now used on 38 % of cultivated hectares
in the Peace River region of Alberta (Anonymous 1997~). Each of these farming methods
will be dealt with separately in this section.
4.11 Conventional TiiIage
Conventional tillage is the traditional method of farming in which fields are tilled
once or twice in the fall after harvest and two to three more tirnes in the spring pnor to
seeding. Nitrogen fertilizer and pre-emergent herbicides are often applied during one of
the tillage operations in the fall. Summerfdlow, the practise of keeping land tilled during
the growing season, is still often included in cropping rotations. This method of famiing
has been widely practised in the Peace River region since it was pioneered shortly after
the turn of the century but like the rest of Alberta, it is slowly being replaced by
conservation tillage because of soil erosion problems, cost of fuel and rnachinery and
time constraints as fewer and fewer growers f m larger and larger tracts of land.
Tillage is one of the most effective ways to reduce and control many soil-borne
plant pathogens. Tillage improves aeration of the soil, speeds up drykg, incorporates
crop residue, and reduces the leveI of some soil pathogens by decomposition. By
incorporating crop residue into the soil, tillage also prevents the spread of spores by wind
or min splash and thus reduces the spread of many foliar diseases. 'The more often a soi1
is tilled, the more enhanced are these effects (Howard 198 1).
Summerfallow involves repeated tillage through the growing season to prevent
weed growth and conserve moisture. Other benefits include faster decomposition of crop
residue and consequently an increased reduction in the number of soil pathogens.
Plowing in the fa11 after harvest will bury any pathogen inoculum that is on crop
debris. Moldboard plowing reduced populations of Rhizoctonia s o h i in the top 5 - 7.5
87
cm of soil (Rothrock 1992). Deep plowing to bury sclerotia to a depth of at least 10 cm is
recornmended for control of sclerotinia rot. However, any additional tÎllage within a
season d l retuni sclerotia back to the soil surface before they have had t h e to
decompose and negate the benefit of plowing for control (Bailey 1996). Plowing or deep
tillage is a common control recommendation for diseases such as ascochyîa blight,
powdery mildew, downy mildew and sclerotinia rot (Howard 198 1 ; Hagedorn 1984).
Tillage also causes the soi1 to w m up faster and dry out more quickly (Rothrock
1992; Green et al 1994). Higher soil temperatures are beneficial for quicker germination
and emergence of pea seed, and reduce the chance of infection by soil pathogens such as
Pythium spp. (Hwang et al. 1997). Lower soil moisture also decreases the risk of
infection by soil paîhogens such as Pythium spp., Rhizoctonin solani and Fusarium spp.
al1 of which are favoured by high soi1 moisture. The incidence of infection by Pythiurn
ultimurn in peas was less when soi1 moisture was low (Rothrock 1992) and
chlamydospore germination of Fusariurn solani Esp. pisi in the rhizosphere of pea seed
was lower when soi1 water is reduced (Cook and Flentje 1967).
Deep tillage or sub-soiling to break up compacted Iayers in hardpan soils is also
beneficial in reducing the incidence and seventy of Fusarium spp. (Tu 1994). When
tillage is removed from a fa-g operation, one of the best cultural practices for control
of diseases is lost.
4.12 Conservation Tiüage
Conservation tillage is any tillage system that leaves at least 30 % of the soil
surface covered by crop residue after seeding (Rothrock 1992). There are two types of
88
conservation tillage systems in use at the present time in western Canada. One system,
known as high disturbance or minimum-tillage, usually involves a tillage pass pnor to
seeding in order to band fertilizer or apply soil incorporated pre-emergent herbicides.
This system generally uses sweeps on an air seeder which causes some mixing of soil and
crop residue at the time of seeding. It does, however, leave more than 30 % of crop
residue on the soil surface. Some aeration and warming of the soil occurs wiîh this
system.
The other system, Eaiown as low disturbance or zero-tillage, is based on disturbing
the soil as little as possible during seeding. It generally uses a coulter or knife system on
the seeder and leaves most of the soil and crop residue undisturbed.
High disturbance tillage accounts for 29.4 % of the cultivated hectares in the
Peace region while low disturbance accounts for only 8.6 ?h of cultivated hectares
(Anonymous 1 997c).
Peas are well suited to conservation tillage. Yields are equal or fiigher than when
grown under conventional tillage. Yields of peas grown under zero-tillage at Fort
Vermilion, AB and Indian Head, SIC were 17% and 9% higher, respectively, dian peas
grown using conventional tillage. Peas grown under zero-tillage had 4% higher L O00
seed weight than when grown under conventional tillage at both Fort Vermilion and
Indian Head (Clayton et al. 1993). Nitrogen fixation was 3 1% higher in peas grown
under zero-tillage as compared to peas grown under conventional tillage (Matus et al.
1997).
Peas are often the h t crop that fmers will grow when making the switch to a
89
conservation tillage system. Given this information, it is not surprishg to find that 70 %
of fields surveyed in this project were f m e d under reduced tillage (40 % had minimum-
ti1lage and 30 % had zero-tillage).
Crop residue on the soil surface provides a favourable environment for both soil
and residue borne pathogens. It is generally believed that crop residue decomposes more
slowly when lefi on the soil surface than when incorporated into the soil (Bailey et al.
1997). This has an impact on sumival of some pathogens. Twenty-four percent more
sclerotia of Sclerotinia sclerotiom, for example, have been shown to survive on the soil
surface than when buried in the soil (Bailey 1996).
Of the few studies that have been done on pea diseases under conservation tillage,
most have not shown disease to be any more of a problem under reduced tillage than
conventional tillage. Bailey et al. (1997) found that although severity of hiycosphaerella
pinodes was lowa under conventional tillage than reduced tillage, there was no effect on
the final yield of peas. They also found that although root rot in peas caused by Fusarizcm
spp. was sometunes lower under conventional tillage, &ere was little difference at
maturity between conventional tillage, minimum-tillage and zero-tillage. Similarly,
Gossan et al. (1996) f o n d no consistent effects of tillage on severity of root rot
(Ftcsarium spp .) or foliar blight (M. pinodes) in peas over the course of their 4-year
study. Turkington et al. (1 997), on the other hand, found that root rot severity of peas was
significantiy higher under conventional mage than under zero-tillage in their 4-year
study at Fort Vermilion in the Peace River region of Alberta.
Although previously rnentioned that sderotia of S. sclerotiomm have higher
90
survivd rates when Ieft on the soi1 surface, other studies have shown that survival of
sclerotia decreases with increasing soil moisture (Teo et al. 1989; Nasser et al. 1995).
Since soil moisture is always higher under conservation mage (Lafond et al. 1992;
Rothrock 1992; Arshad & Gill 1996; Bailey 1 W6), this should Iead to increased
degradation of sclerotia and lower survivability. Bailey (1996) States that any heavy
residue from a non-host crop will increase soil moisture and microbial activity which will
Iead to decreased survivability of sclerotia. She concluded that there should be no greater
risk of sclerotinia rot with minimum-tillage or zero-tillage than with conventionai tillage.
As farmers have switched their seeding equipment over to conservation tillage,
many have also increased the row spacing from 15 cm to 23 cm or 30 cm. This was done
to overcome dificulties in seeding through heavy crop residues in the wetter parts of the
prairies (Lafond et al. 1997). Not much research bas been done on row spacing and its
effect on disease. Wider row spacing should help to reduce the levels of some diseases
such as sclerotinia rot by increasing the airflow through the canopy and thereby
decreasing humidity. Lafond et al. (1997) in a sîudy on row spacings concluded that
wider row spacing sometimes decreased disease levels. He also stated, however, that
more research was needed in order to be crop and area specific.
4.13 Other CuItural Practices
Other cultural practices known to affect disease include changes in seeding dates
and seeding depth. Given the short growing season in the Peace region, making much of a
change in seeding dates is difficult. Peas are usually the fust crop to be seeded in the
spnng and research has shown that delaying seeding has a negative e f h t on yield. Both
91
the Alberta and Saskatchewan Pulse Growers Manuals recommend early seeding of peas
to achieve the best yields and quality (Anonymous 1993; Auonymous 1997a). Hwang et
al. (1997) showed that yield of peas decreased significantly with successive seeding dates
in 2 out of 3 years of their research. Early seeding is also recommended to prevent
powdery miIdew fiom causing significant yield and quality losses (Zimrner 1984)- It wiIl
also allow peas to reach a more mature stage before soil temperatures become wann
enough for fusarium wilt to become active.
Seeding depth will have an influence on the amont of time it takes for the pea
seed to germinate and emerge fiom the ground. Germuiating seeds are susceptible to
Pythiurn spp. for only 48 - 72 hours afier planting under good conditions (Harrnan
1984a). n i e deeper the seed is placed in the ground, the cooler the soi1 temperature and
the longer it will take for the seed to gemiinate and emerge fkom the gound (Green et al.
1994). This increases the amount of time that the pea seed is susceptible to infection
(Leach 1947). Current recommendations are to seed peas at a depth of 2.5 cm below
moisture (Clayton G.W. - Personal Communication 1997). For example, if moisture is
2.5 cm below the soi1 surface (top 2.5 cm of soil is dry), peas should be seeded to a depth
of 5.0 cm. This allows for the warmest soi1 temperature and sufficient moisture for rapid
germination and emergence fkom the soil.
4.2 Host Resistance
There are two methods to tramfer resistance to disease in peas. One is wirh
traditional plant breeding methods and the second is with genetic engineering.
92
Large collections of pea germplasm are held at many cenees around the world: Italy,
ICARDA, Syria, Poland, UK, Sweden, USA, Germany and India. Between 1500 and
4000 accessions are held in the majority of these centres (Davies 1993a). Due mainly to
the efforts of Swedish geneticists, more than 2000 mutations of peas are recognized and
of these over 400 developmentai and morphological mutants have been descrïbed (Marx
1977; Davies 1993a).
Traditional plant breeding methods using both wild pea types and mutants have
conferred resistance to many pathogens and diseases (Davies 1993a). Resistance to
Ascschytapisi has been successfully bred into many varieties, but breeding for
resistance to MycosphaereZZa pinodes has been unsuccessful (Cousins 1997). Only very
low Ievels ofresistance have been found in the germplasm evaluated for resistance to M.
pinodes. None of the varieties presently available in western Canada has any resistance to
M. pinodes. Over 30 Pisum genotypes with partial resistance have been identified in a
study at Morden, MB. The use of these genotypes in a breeding program will allow the
pyramiding of genes to give better and more long lasting resistance to M. pinodes (Xue et
al. 1997). Breeders expect the pyramiding of many genes, each with a small effect, to be
a long and labourious task (Johnston & Kutcher 1998). One unique attempt has been the
electrofusion of cells fiom field pea and Lathym spp. which possess high levels of
resistance to the pathogen (Blade 1998). Other traits to be successfully bred into peas
include resistance to downy mildew (caused by Peronospora viciae), powdery mildew
(caused by BIumeriapis& fus- wilt (caused by Fusarium oxysporum f.sp. pisi),
bacteriai blight (caused by Pseudornonas syringae pv. pisi), pea mosaic virus, pea enation
93
mosaic virus, pea feafroll virus, pea seed-borne mosaic virus and pea early browning
virus (Cousin 1997).
Genetic engineering is still in its infancy in field peas. Several research groups are
working on genetically mod img peas. In Canada, the Plant Biotechnology Institute in
Saskatoon and the University of Alberta in Edmonton both have researchers working on
introducing foreign genes into peas (Polowick 1996; Ozga & Reinecke 1998). The
Biotechnology Unit of CSIRO in Canberra, Australia, however, has been the most
successful so far. They have developed transgenic pea lines with resistance to herbicides,
insects, vinises and one line with increased methionine content. Field testing has
occuned in Australia and New Zealand and commercial development should occur in the
next 3 to 4 years (Blade 1998).
Virus resistance is one area that could benefit kom genetic engineering. The
introduction of genes for viral coat protein that simulates cross protection, satellite RNA
which modifies the seventy of symptoms of some viruses, antisense RNA to block RNA
and DNA vinises, ribozymes (short DNA sequences) which can cleave RNA molecules,
and viral replicase sequences are al1 examples of genes that have been successfblly
introduced into other plants (Davies 1993b) to provide resistance to viruses. Al1 of these
could potentially be used to confer Wal resistance in peas.
Opportunities for improvement by using traditional breecling rnethods still exist
for fungai pathogens (Davies 1993b); however, examples exist where plants have been
transformed using foreign genes to provide disease resistance. Chitinase genes fiom
beans (PhaseoZus vulgaris) and the bacterium Senatia marcescens have been introduced
94
into tobacco and canola and a gene for a riboryme-inactivahg protein fkom barley
inserted in tobacco have dl been shown to reduce the severity of Qmping off due to
Rhizoctonia solani. Since beans are also a legume, the likelihood that the chitinase genes
could be introduced into peas are very good. Pathogenesis-related proteins and lectins
could also be traosfered to convey resistance (Davies 1993b). In the near future,
traditional breeding methods may be the best way to improve resistance to fimgal
pathogens in peas. Nevertheless, it is important to remember that although there is plenty
of genetic stock available within the pea family, the time may corne when we will deplete
the useh1 genes fkom peas and foreign genes will be needed to help provide resistance to
fungal pathogens.
4.3 Chernical Controls
The number of fungicides registered for use on peas in western Canada is limited.
At the current time, there are only three fungicides for use as seed treatments and two as
foliar sprays. Thiram (Thhm 75WP) and captan (Captan FL) are registered as seed
treatments for control of seed decay, seedling blight, damping off and root rot and
metalaxyl (Apron FL) is registered for control of seed rot and seedling blight caused by
Pythiurn spp. Thiram and metalaxyl c m be mixed together to give a broader spectnim of
protection (Anonymous 1 998a).
Chlorothalonil (Bravo 500) is a protectant fungicide for the control of
mycosphaerella blight. It is applied at the first sign of disease spp toms and can be
repeated up to three times at 10-14 day intervals if weather conditions are conducive to
95
development of the disease (Anonymous 1 998a).
Sulphur (Kumulus DF) is registered for the control of powdery mildew. It can be
applied at the f k t appearance of the disease and repeated at 7 to 10 day intervals if
needed (Anonymous 1998b).
4.4 Proposed Pea Disease Management Program
1. Crop Rotation
Crop rotation is still one of the best means of avoiding problems with disease. It
lengthens the t h e between susceptible crops, allowing the pathogen population
to decline to levels that prevent significant yield losses. Crop rotation is even
more important under conservation tillage than under conventional tillage since
the benefits of tillage for disease control are lost. Growing peas only once every 4
to 5-years will reduce inoculum levels of rnost pathogens to levels where they
will no longer cause econornic damage. Davies (199 1) reported a higher incidence
of sclerotinia stem rot in canoIa when peas were included in the rotation.
Therefore, having 1 or 2 years of non-host crops between canola and peas is
important. A crop rotation consisting of cereal-oilseed-cereal-peas would be an
ideal rotation to rninimize disease problems in the crops grown in the Peace River
region.
2. Plowing
Deep plowing (10 - 15 cm) in the fa11 will bury inoculum of many pathogens and
is recommended if diseases such as mycosphaerella blight, sclerotinia rot,
p o w d q rnildew and downy mildew are present in the pea crop. Any additional
tillage, however, will return sclerotia of sclerotinia rot to the surface and negate
the effects of plowing. Unfortunately, the option of plowing is only available to
growers who are not practising conservation tillage.
3. Use of High Vigour Disease-Free Seed
Pea seed should have hi& germination, high vigour and be disease-fkee. Seed
grown in drier parts of the province is preferable since it is usually free of
pathogens. The benefits of using high vigour seed are numerous. Seeds germinate
more quickly and are better able to withstand stressfùl conditions such as cool,
wet soils. Pea seed can be vigour tested while being checked for germination at a
number of seed testing labs in Alberta.
Elec~cal conductivity is used to mesure vigour in peas and is expressed in
micro-siemens (JAS). A score of 0-20 ,US means that peas are very vigorous and
are suitable for early seeding into cool soils. A score of 20-30 / IS Uidicates
medium vigour. These peas may not perfom as well under adverse conditions but
should still be suitable for later seeding into warm soils. A score over 30 ,US
indicates low to very low vigour and use of these peas for seed is not
recommended (Foster-Stubbs 1997).
4. Seed Treatment
Seed treatment will prevent the introduction of seed-borne pathogens fkom
diseases such as mycosphaerella blight. It will also provide protection against soi1
pathogens such as Pythium spp., Fusarium spp. and Rhizoctonia soluni as the
seed geminates and emerges &om the ground. Hwang et al. (1 997) showed that
significantly higher yields were obtained when pea seed was treated than when
left untreated.
A number of fungicides are registered for use on peas for root diseases. Thiram
98
and captan will provide protection against seed decay, root rot, h p i n g off and
seedling blight, while metalaxyl gives protection against seed rots and seedling
blights caused by Pythium spp. Thiram and metalaxyl can be mixed to give a
broader s p e c m of protection (Anonymous 1 998a).
5. Seeding Depth
Peas should be seeded at a depth of 2.5 cm below moisture. This allows for the
wannest soil temperature and sufficient moisture for quick germination and rapid
emergence.
6. Lime if Seeding on Acid Soils
Since a large portion of the Peace River region has acid soils (Anonymous
1996~). liming to increase the pH to between 6.0 and 6.4 should help to decrease
the severiîy of root rot (Tu 1987b). Liming also has many other benefits such as
increasing yield (Arshad & Gill 1996), creating a more favourable environment
for soil microbiological activity, increasing the availability of several nutrients
and improving the physical properties of some soils (Anonymous 1996~).
7. Subsoil if Seeding on Hardpan Soils
Hardpan soils, consisting of a compacted layer of soi1 found at the depth of
tillage, are created by soil compaction fiom numerous heavy rnachinery passes
(Greevers and Boehm 1994) and are found in many Race River region fields.
Subsoiling to break up the hardpan will help to reduce both root rot incidence and
severity (Tu 1987b). Yield may also be positively afZected. Coy (1 985) in a
subsoiling study near Grande Prairie, AB showed that yields of wheat were
always higher in subsoiled plots than in non-subsoiled plots.
8. Use Foliar Funacides if Needed
Chlorothalonil (Bravo 500) has recently been registered for control of
mycosphaerella blight in peas. Pea plants should be checked regularly at earIy
flowering for signs of mycosphaerella blight and if symptoms appear, an
application of chlorothalonil can be made. If weather conditions are favourable
for development of the disease (warm and humid), applications can be repeated at
10 - 14 day intervals. A total of 3 applications per season c m be made if
necessary (Anonymous 2 998a).
Although powdery mildew was not reported in this survey, it has previously been
reported in the Peace River region (Hanison & Lafiamme 1996). If an early
infection of powdery mildew is noticed, sulphur (Kumulus DF) can be sprayed to
prevent senous damage to the pea crop. It is applied when the disease is e s t
noticed and can be repeated at 7 -10 &y intervaIs if needed. If the infection
occurs late, a fungicide is not required since no economic damage will occur to
the peas.
9. Avoid Seeding Next to Last Year's Pea Fields
Since symptoms of mycosphaerella blight were found in al1 pea fields surveyed in
1997, it can be assumed that this disease is widespread in the Peace River region.
Therefore, since ascospores c m be blown by wind for a considerable distance,
peas should not be seeded next to a field that had peas in it in the previous year.
1 O0
10. Seed Peas as Early as Possible
Peas shouid be the first crop in the ground in the Peace River region. Planting
peas as early as possible wiIl prevent serious damage Eom powdery mildew and
fusarium wilt, both of which are favoured by high temperahires. It will also
increase the possibility of obtaining the highest yields and quality.
Chapter 5 - Conclusions and Recommendations
Root rot, mycosphaerella blight and sclerotinia rot are the main diseases of field
peas in the Peace River region of Alberta. Other diseases may be present but are not yet
causing significant losses to growers in the region. It is, however, important to keep these
other diseases in rnind when planning a successful disease control program.
Kraft (1 99 1) States that roo t rot is the principle limiting factor to increasing or
stabilizing yields of peas in North America. The results of this study would agree with
this statement. Root rot is the disease with the potential to have the most serious effect on
viability of pea growing for this region. Although 1997 was a year of above normal
precipitation, and this may have been a cause for unusudly high incidence and severity of
root rot, there is research to show that root rot severity may even be worse in dry years
due to darnage to the root system (Tu 1994). Seed treatment will help to minimize the
effects of root rot but only provides protection for a shoa time during germination and
emergence of the peas. Other factors such as a 4 to 5-year rotation between pea crops, use
of high vigour seed, subsoiling, liming, and avoiding stresshl phenoxy herbicides such as
MCPA and MCPB will d l aid in reducing the effects of root rot.
Mycosphaerella blight also has the potential to become a serious disease of peas in
this region. The disease is very moisture dependent and may not be a problem in dner
years. However, the Peace River region is better hown for having excess moisture rather
than being deficient in moisture. A newly registered fungicide, chlorothdonil (Bravo
5ûû), is now available for controlling mycosphaerella blight in peas. This should help to
102
keep the disease fiom causing serious losses in high moisture years. Much research is
also being done in western Canada to breed resistance to this disease in pea varieties but
resistant varieties are not yet available. For the present t h e , rotation, seed treatment and
the use of f o l k fungicides will be very important means of keeping this disease in check.
Sclerotinia rot is not considered a serious disease in peas since it does not cause
significant yield losses (Johnston & Kutcher 1998). It is, however, a concern to any
grower who has canoIa in his rotation since it adds inocdum to the soil. Because there
are no fimgicides currently registered for the control of sclerotinia rot in peas, and since
the incidence of sclerotinia stem rot in canola is always higher when peas are in the
rotation, producers who grow both peas and canola may have to add a k g i c i d e
application for sclerotinia control in canola to prevent sclerotinia stem rot fiom becoming
a yield reducing disease in canola.
Awareness of these pea disease management strategies d l allow peas to
continue being a high yielding, viable crop for this region.
Given the resources available, I consider this survey to have been successful in
achieving the objectives of this project. The identification of pathogens was not as
successful as it could have been but other parts of the project gave some very interesting
results.
In the hope that any future surveys of the Peace Riverregion are more successful,
1 make the following recommendations:
1. Sample only one area at a thne. The Peace River region is very large and driving
the distances involved is costly and time connimhg. It would have been better to
103
have picked one area in the region and done the sampling, identification and
culturing more quicWy and at much less cost. There would have been less plants
to handle and more t h e to devote to the cultures and identification. The only
other option is to have more people helping if sampling of the whole region is to
be done.
2. Use selective media for a number of known pathogens. The visual identification
of symptoms wili give a clue as to what selective media should be used.
3. Do as much of the visuai identification of symptoms as possible in the field.
Again, because of the size of the Peace River region and time constraints this is
difficult to do unless a lot of people are available to help. n i e grower should also
be invited dong since most of them are very interested in what is happening to
theû pea crop. This could also involve more time since the f m e r will be asking
a lot of questions. From an extension point of view, however, this would be a very
valuable exercise. Another good reason to pick only one area instead of the whole
region.
Appendix 1
1997 Peace Region Field Pea Disease Survey
Name of Cooperator:
Ad dress:
Phone 3:
LegaI Land Location of Field:
1. How many acres of field peas are you growing in I997? acres.
2. How many years have you been growùig fieId peas? O 1st time C l 1 year Q 2 years Cl 3 or more years
3. What has been the cropping history of this field?
4. Have you had any diseases in your peas in previous years? I f yes, did you have it identified?
D no O yes Disease (s):
5. Did you use cenified or beîter seed? I f yes, what variety of peas is seeded in this field?
no O yes Variety: -
6. Did you use seed treatment? I f yes, please indicate which one you used. O no D Y = Cl Thiram 75 WP
O Captan FL O Apron FL O Other
7. Did you use inoculant? If yes, what Spe was used? il no O yes fl peat powder
17 liquid 17 granular
8. Did you apply any fertilizer? If yes, please indicate type, rate and meîhod of application? i l no 0 YS
Type:
Rate:
Method of Application: Eî broadcast CI banded iJ with the seed
9. Were yow peas zero-tilled? 17 yes If yes, how many years has this field been zero-tilled?
O 1st time El 1 year 172years O 3 or more years
O no If no, how many tillage passes were made pnor to seeding? Cl one tillage pass !l two tillage passes 0 three or more tillage passes
10. Did you use a land roller after seeding? If yes, when did you roll the land? Cl no O yes Ci immediately after seeding
Cf before ernergence of peas O after emergence of peas
1 1. Have you used any herbicides on this pea field? If yes, please indicate type and rate used. Cl no I l yes
Herbicide [sj used: Rate used:
12. What herbicides were used in tbis field in the last two years?
1996: Herbicide (s) used:
1995: Herbicide (s) used:
Rate used:
Rate used:
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