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Interactions between drought and forest pests and diseases
3 approaches
Epidemiological approach: field surveys
Experimental, quantitative approach: metaanalysis of available evidence
Analysis of underlying mechanisms in experimental conditions
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Field surveys
Field surveys by operational services: spatial and temporal patterns of occurrence and damagefor different (important) species
Qualitative information (expert judgement) availabe quickly: massive bark beetle attack in 2003 (increase after a decrease in 2002 in the areas damaged by the 1999 storms, and in other regions), etc.
Quantitative information on occurrence / damage is delayed (with a few exceptions) due to validation procedure, not homogeneous over the borders, and generally poor quality.
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Field surveys and their interpretation
Relating intensity of damage to intensity of water stress / heat is sofar almost impossible outside experimental conditions
Few scientists have tried to explore the (large-scale) spatial-temporal patterns of pests and diseases versus climate stress
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Analysis of published information on the response of pests and diseases to
water stressSee Jactel et al.
Numerous experiments, few well documented!
Damage proportional to pests and diseases
But : response of pests and diseases to stress is non linear (confirms that the intensity of water stress must be known)
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Working group
Marie-Laure Desprez-Loustau, Dominique Piou, Andrea Vannini, Louis-Michel Nageleisen and Benoît Marçais
Interactive effects of drought and pathogens in forest tress
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THE DISEASE TRIANGLE
PATHOGEN HOST
DISEASE
ENVIRONMENT natural and anthropic effects ; local and global change
(drought, heat, sylviculture, …)
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DIRECT or INDIRECT EFFECTS OF DROUGHT ON PATHOGENS?
Foliar polycyclic diseases : often less common and less severe during drought episodes:
rain/moisture needed for dissemination, germination and penetration)
E
DISEASE
HP
Relative importance of external / internal infection processes:
Stem and root diseases : indirect effects through host physiology : water stress is determinant
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1. Effects of drought on host –pathogen interaction :
increased susceptibility to pathogens:PREDISPOSITION
• generally, development of disease during or following stress
• some types of pathogens: mainly necrotrophs, facultative parasites
E
HP
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E
HP
2. Effects of disease on the plant response to drought stress :
decreased tolerance to MULTIPLE STRESSES
•disease developed before stress
•all types of pathogens (including biotrophs)
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1. Predisposition of trees to disease by drought (heat) stress
« Drought-enhanced diseases » : favoured by drought but caused by « true parasites »
mainly cankers (necrotroph pathogens)Example : Sphaeropsis sapinea
Sensu lato: Increased « disease proneness » resulting from external causes (does not imply that infection always occurs after stress)
« Drought-induced diseases »: caused by opportunistic, facultative or conditional parasites
mainly endophytic speciesExample : Biscognauxia mediterranea
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Mechanisms of predisposition
Decrease in photosynthetic activity and altered protein synthesis in dehydrated plants resulting in :
Metabolic changes : improved substrate for pathogen (nitrogen)
Decreased defensive compounds (phytoalexins, enzymes, etc…)
Slowed defenses limiting compartmentalization
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Experimental evidence of predisposition mechanisms: Effects of water stress on the rate of formation of
physical defenses (after wounding) in Abies grandis* (Puritch & Mulklick 1975)
0
10
20
30
40
50
60
70
0 1 2 3 4
Stress level (-MPa)
Nb
Day
s to
fo
rm N
IT
exp1exp2exp3exp4
* NIT = non-suberized impervious tissue, involved in necrophylactic periderm formation
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Sphaeropsis sapinea : an endemic pathogen in Europe with recent outbreaks in pine forests
associated with predisposing stresses
1991-4 D
1996 D
1990-94 D t°>
1980's
1984 t°>
1986 D t°> t°<
2002
1982-5 D N
1990
1999 H
1990-2 D
2001-2 t°>
1988
2000-1
1986-90
1994 D
1992
1998
1986
1994 D t°>
1994 H1986-9 D
2001 D
1992-3 D
1991
1994
1996-97 D t°>
P. pinasterP. halepensis
P. radiataP. nigraP. pinea
P. sylvestris
1991-4 D
1996 D
1990-94 D t°>
1980's
1984 t°>
1986 D t°> t°<
2002
1982-5 D N
1990
1999 H
1990-2 D
2001-2 t°>
1988
2000-1
1986-90
1994 D
1992
1998
1986
1994 D t°>
1994 H1986-9 D
2001 D
1992-3 D
1991
1994
1996-97 D t°>
P. pinasterP. halepensis
P. radiataP. nigraP. pinea
P. sylvestris
P. pinasterP. pinasterP. halepensisP. halepensis
P. radiataP. radiataP. nigraP. nigraP. pineaP. pinea
P. sylvestrisP. sylvestris
Stress factors : N = nitrogen pollutψantsD = drought t°> = hight temperaturesH = Hail t°< = low temperatures
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Main symptoms associated with drought * S. sapinea
Bark necrosisCanker (uncommon in Europe)
Crown or branch dieback
Affected stands are always plantation forests(P. pinaster not affected in Europe but heavily damaged in S Africa
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Biscogniauxia (Hypoxylon) mediterraneum :an endophyte turning to parasite with drought stress
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Q. cerris
Q. pubescens
5101520253035
-4,0 -3,5 -3,0 -2,5 -2,0 -1,5 -1,0
MWP (MPa)
Iso
lati
on
%B. mediterraneum is morereadily isolated in stressed
trees (Quercus cerris) (Lucero 2000)
The development of symptoms is linked to water status
(Vannini et al. 1996)
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1. Predisposition2. Combination of biotic and abiotic stresses : effects of multiples stresses
Both infection and drought act as stresses on the plant
In general, this results in additive or synergistic deleterious effects
3.Collapse or exhaustion
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Combined effects of infection and drought stresses1. Root pathogen (affecting directly water relations)
Ex : Collybia fusipes, root pathogen on oak (Marçais)
Not a predisposition effect : oaks growing in high sand content are not more susceptible to Collybia) > likely explained by a reduced water uptake due to root loss
0123456789
Sand content (%)0 20 40 60
Rel
ativ
e ris
k of
dec
line
asso
ciat
ed w
ith in
fect
ion
0123456789
Sand content (%)0 20 40 60
Rel
ativ
e ris
k of
dec
line
asso
ciat
ed w
ith in
fect
ion
0123456789
0123456789
Sand content (%)0 20 40 60
Rel
ativ
e ris
k of
dec
line
asso
ciat
ed w
ith in
fect
ion risk of decline in
infected trees (compared to healthy ones) increases sign. with sand content (decreasing water availability)
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2. Foliar pathogen (Groundsel rust, Ayres 1991)
5
9
13
17
21
9 11 13 15
Days after inoculation
Pn (m
g CO
2 pl
ant-1
h-1
)
HEALTHY
INF
DROUGHT
INF +DROUGHT
-0.2 MPa
-1.6 MPa
-1.1 MPa
-0.8 MPa
(affecting the photosynthesis)
drought prevents the growth of new, uninfected leaves
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The decline concept (Manion 1991)
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Some documented oak declines in FranceTronçais Harth Mediterr.
regionHaguenau Lure
SpeciesMortality period
Q. robur
1978-82
Q.spp
1991-94
Q.suber
1991-94
Q.robur, Q.petraea1995-97
Q.robur
1995-97PredisposingSite, AgeRoot pathogens (Collybia, Phytophthora)
+
+
+
(+)
+
+
+ +
(+)
IncitingWater stressDefoliatorsPowdery mildew
+(76)??
+(89-92)(+)
+(89-92)(+)
?+(93-94)+(93-94)
+(89-91)
ContributoryFungi (Armillaria)Insects(borers)
+
? +
+
+ + +
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MECHANISMS : Plants stress responses
STRESSStress recognition
Signal transduction
Gene expression
Altered cell metabolism
Physiological and developmental response
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Effects of stresses in plant populations and communities
Drought and infection can have additive effects in increasing both inter- and intra- specific competitive fitness
Competition for resources
DROUGHTDISEASE
Fitness
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Competitive fitness affected by disease and droughtgroundsel rust: an experimental demonstration of
(Ayres 1991)
Dry
wei
ght
Shoot potential
Well-watered
Water-stressed
Monoculture H or I (no competition)
Mixture H + I (competition)
�Healthy
Infected Reallocation of
the ressources (esp. Water) within the pop. In favour of the healthy ind.Negative effect of double stress less than at the individual level
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An example of long-term response of ecosystems to extreme stresses
0
20
40
60
80
100
1930 1940 1950 1960 1970 1980
Effects of chestnut blight (Cryphonectria parasitica) on a deciduous forest ecosystem (after Day & Monk 1974)
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Conclusions and future prospects
Interactive effects of drought and disease favor two types of damage :
drought-induced diseases (Predisposition) declines (Multiple stress effects)
In both cases, the severity of water stressexperienced by trees is crucial
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Conclusions and future prospects (2)
Considerations for management
Measures aimed at limiting pathogen spread : seeds (S. sapinea), nursery plants (Phytophthoraspp)
Favour silvicultural practices that promote elastic responses of stands to drought(heat) stresses (cf physiology group), including use of inter- and intra- specific diversity
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Conclusions and future prospects (3)
Future needs for research
Heat stress effects
Long-term effects of multiple stresses (ex :N deposition * drought * disease)
Shifts between mutualism and parasitism : genetic and environmental control
Population (community)-wide studies of the impact of multiple stresses (on yield and diversity) ; implications for breeding programs and sylviculture