assessing the reaction of american wildrice to inoculated pathogens
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Assessing the Reaction of American Wildrice to Inoculated Pathogens. Raymond Porter*, Robert Nyvall, and Laura Carey University of Minnesota, NCROC, 1861 E. Hwy 169, Grand Rapids, MN 55744. photos by James Percich, U of MN. - PowerPoint PPT PresentationTRANSCRIPT
Assessing the Reaction of American Wildrice to Inoculated Pathogens.Assessing the Reaction of American Wildrice to Inoculated Pathogens.Raymond Porter*, Robert Nyvall, and Laura CareyRaymond Porter*, Robert Nyvall, and Laura CareyUniversity of Minnesota, NCROC, 1861 E. Hwy 169, Grand Rapids, MN 55744University of Minnesota, NCROC, 1861 E. Hwy 169, Grand Rapids, MN 55744
INTRODUCTION
American wildrice (Zizania palustris— Fig. 1) has several
known stem rot diseases, caused by the fungal
pathogens Nakataea sigmoidea (Fig. 2), Bipolaris oryzae,
and B. sorokiniana. The Bipolaris species also cause the
foliar diseases fungal brown spot and spot blotch (Fig. 3).
Since these diseases have previously been the target of
selection to improve varietal resistance, methods have
been developed to inoculate leaves with conidia
suspensions to develop resistant varieties. Selection for
resistance to stem rots in wildrice has not been done.
In this study, methods were developed for growing
inoculum of N. sigmoidea and for delivering known
quantities of propagules of all three species to the stems
of wildrice varieties. These methods were applied in a
multilocation trial, where plots of a single variety were
inoculated with all three species to test several fungicides
for efficacy in reducing the effects of these diseases. This
experiment was used as an opportunity to develop
methods to quantify differences in disease severity for
future assessment of varietal resistance in a breeding
program. Specific objectives included:
1) Develop and assess the effectiveness of a method of
production and inoculation of N. sigmoidea, and
2) Assess the effectiveness of leaf and stem disease
rating methods in detecting statistical differences
produced by different treatments (fungicides).
MATERIALS AND METHODS
Variety: Itasca (a high-yielding, shattering resistant
variety selected for resistance to fungal brown spot.
Design: RCBD, 1.5 x 3.0 m plots, 8 fungicide treatments,
6 reps at each of 3 locations.
Inoculation and treatments: Eighteen isolates of B.
sorokiniana were cultured on a medium consisting of
course perlite, wildrice flour, and 1% PDA (1:2:4).
Separately, 18 isolates of B. oryzae were cultured on
same medium. Conidia were filtered into suspension
with water after 15 (B. sorokiniana) or 18 (B. oryzae)
days of growth. Inoculum of B. sorokiniana was applied
at a rate of 4.5 million conidia per plot with a CO2
sprayer at mid-tillering. B. oryzae was applied 5 days
later at 1.8 million conidia per plot.
Six N. sigmoidea isolates were cultured on the same
medium. Mature sclerotia were produced in 14 days, at
which time medium was dried, crushed, and filtered
through a #10 seive (Fig. 4) to produce the dried
inoculum (750 sclerotia/ml). At late tillering, inoculum
was spread with a Spred-Rite granular applicator on the
water surface just inside the side borders of each plot,
at 16,000 sclerotia per plot (Fig. 5).
Five fungicides—Tilt (propaconizole), Headline
(pyraclostrobin), Quadris (azoxystrobin), Quilt
(azoxystrobin+propaconizole), and Stratego
(trifloxystrobin+propaconizole) were applied at boot
stage, and two of these were also applied at heading,
making up seven fungicide-timing treatments plus a
control.
Disease assessment and analysis:
All plots at a location were harvested on the same
day. Ten stems and 20 flag leaves per plot were
collected at harvest and frozen to be later rated for
area affected by disease. Diseased leaf area was
estimated by comparison with the Clive James key
for Septoria leaf blotch (Key 1.6.1). Diseased stem
area was estimated without the aid of a key. Two
pieces were cultured from each stem and one piece
from each leaf (a total of 20 each of stem and leaf
pieces). Five to seven days later, fungal species were
identified for each lesion to estimate the incidence of
each pathogen. Indexes were calculated for leaf and
stem diseased area due to a particular pathogen by
multiplying the visual estimate by the frequency of
the pathogen incidence on cultured leaf pieces.
Each location was analyzed separately using SAS
Proc Mixed, adjusting for spatial variability with the
function sp(powa). Significance of pairwise
comparisons at P<0.05 was used as the criterion for
declaring treatments statistically different.
Fig. 1 Cultivated American wildrice, Zizania palustris cv. 'Petrowske Purple'
Fig. 2 Sclerotia inside of a wildrice stem infected with Nakataea sigmoidea
Fig. 3 Fungal brown spot (left), caused by B. oryzae, and Spot blotch caused by B. sorokiniana.
Fig. 4 Granular inoculum of N. sigmoidea in perlite medium to be filtered (left). Sclerotia amid perlite granules in inoculum (right).
Fig. 5 Granular applicator used to apply dry inoculum of N. sigmoidea (left). Granules on water surface adhering to wildrice stem (circled in photo on right).
Fig. 6 Estimated diseased leaf area of flag leaves for treatments at Aitkin. Treatments having a letter in common are not significantly different at P=0.05.
RESULTS
Significant treatment differences were found for
diseased leaf area at all three locations, but the Aitkin
location was especially severe, with many significant
comparisons (Fig. 6). Incidence of B. oryzae was far
greater than B. sorokiniana in leaf lesions and also
showed significant differences (not shown). When
multiplied by diseased leaf area, the index magnified
treatment differences (Fig. 7).
Fig. 7 Index estimating percentage of leaf area diseased due to B. oryzae at Aitkin.
photo by Dave Hansen, MAES
Stem lesion area was difficult to estimate, and
analysis showed no significant treatment differences.
B. oryzae was predominant among the stem lesions,
and N. sigmoidea did not have as high an incidence
as expected (1-3% vs 10-30% for B. oryzae). However,
when stem lesion area was multiplied by incidence of
all three stem pathogens combined, significant
treatment differences were seen (Fig. 8).
Fig. 8 Index estimating percentage of stem area diseased due to Bipolaris spp. and N. sigmoidea at Aitkin.
CONCLUSIONS
1. Because incidence of Nakatae in stems was low,
this method of inoculation needs to be improved.
Earlier application of inoculum and development
of a method to apply sclerotia directly to stems are
priorities for future research.
2. Indexing of both diseased leaf and stem area with
pathogen incidence in cultured lesions improves
statistical discrimination between treatments. This
may indicate that a significant number of lesions
(both stem and leaf) are of non-fungal origin.
ACKNOWLEDGMENTS
Funded through USDA-ARS Cooperative Agreement No. 58-
3640-4-122. Wildrice growers Tom Godward, Rod Skoe, and Ed
Mohs graciously provided space for experiments. Dan Braaten
and Henry Schumer provided technical assistance.
Representatives of BASF, Bayer, and Syngenta provided
fungicides for the treatments.
photos by James Percich, U of MN
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