Impact of Preceding Crops on Incidence and Severity of Disease in Canola
Brian Jenks, Shanna Mazurek, Gary Willoughby
North Central Research Extension Center, Minot, North Dakota
Art Lamey, North Dakota State University, Fargo, North Dakota
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INTRODUCTION
Low crop prices and severe disease problems
have forced many spring wheat producers in the northern plains to turn to
alternative broadleaf crops. For
example, canola acres have increased from 18,000 acres in 1991 to 850,000 acres
in 1999 in North Dakota. Current
recommendations are to plant a broadleaf crop like canola or sunflower no more
than once every four years to avoid buildup of disease inoculum.
However, producers want rotations involving more profitable broadleaf
crops to be shortened. Some
producers have planted a broadleaf crop for two consecutive years on the same
field in an attempt to increase overall profit potential and stay in business
another season. Additional
information on the impact of crop rotation on disease will help producers
optimize their limited resources.
The objectives of this research were to:
document the influence of crop rotation on the incidence and severity of
sclerotinia, blackleg, and alternaria black spot in canola and flax; determine
the impact of the previous crop on disease levels in canola; and determine if
fungicide applications can be eliminated or rates reduced by altering the
sequence of crops in the rotation.
MATERIALS
AND METHODS
Eighteen treatments consisting of 6 crop
rotations were established in 2000 (Table 1).
The rotations consist of canola every year, every other year, every third
year, and every fourth year. Every
crop of the rotation will be raised each year to explain the effect of
individual years. There will be 7
canola, 5 wheat, 4 barley, and 2 flax plots, replicated 4 times, each year.
The study will be conducted a minimum of four years to observe the
effects of a four year rotation. The
plots are 30 by 180 feet with a 30 ft border around each plot.
One-half
of each canola plot was treated with a fungicide.
Ronilan® at 12 fl oz was applied June 30 at approximately 20% bloom.
The other half of the plot was left untreated for comparison.
Two methods of sampling were used to evaluate
each canola subplot for sclerotinia stem rot (SSR) disease risk at 20% bloom and
1 week later. Petals were collected
from four areas in each subplot (Morrall and Thomson 1991).
Four petals from each sample were plated on a semi-selective media for a
total of 16 petals per subplot. The
second method involved placing a culture plate of SM semi-selective medium (Steadman
et al. 1994) on the ground in four places in the subplot.
The culture plates were left uncovered in the plot for 2.5 hours.
Culture plates from both tests were incubated in the dark at room
temperature for 3 days. Only 56
Steadman plates were available for the second exposure date, therefore, only 2
plates were exposed in each untreated half of the plots.
Disease incidence and severity were
evaluated August 9 for both SSR and blackleg. Standing plants adjacent to canola
swaths were evaluated for both SSR and blackleg.
A total of 100 plants per subplot, 10 plants in 10 locations, were
evaluated. Disease severity was
rated on a scale of 0 to 5, 0 being no disease and 5 being total girdling of
stem resulting in death of plant.
One-half of each wheat and barley plot was also treated with fungicide.
Folicur at 4 fl oz was applied July 10 for Fusarium head blight control.
Fusarium incidence and severity were evaluated July 25 by collecting 20
heads per subplot and rated based on percent of head infected.
RESULTS
AND DISCUSSION
Sclerotinia ascospore levels indicated a
very low disease risk. The first
petal test indicated only 3% incidence of sclerotinia spores on the canola
petals sampled. The second test,
one week later, showed an average of 22% incidence among all the canola plots,
which was still considered low risk (Morrall and Thomson 1991).
The Steadman test also indicated low levels
of ascospores. Only 18 of the 224
plates exposed on the first date tested positive for sclerotinia, which
indicates an 8% incidence. There
were only 4 of the 56 plates that tested positive for sclerotinia in the second
exposure for an incidence of 7%. Although
both tests indicated low disease risk, they are actually measuring different
spore loads. The petal test
primarily measured spores that have been blown in from neighboring fields,
whereas the Steadman test, which was exposed at ground level, selected for
ascospores produced within the individual plots.
Differences between crop rotations could
not be detected yet because the entire research area was in durum in 1999.
However, SSR incidence and severity were lower where fungicide was
applied compared to no fungicide application.
The average plot severity for all of the treated plots was 0.05 compared
with 0.12 in the untreated plots. The
average disease incidence for the treated plots was 2% compared with 5% in the
untreated plots. Black leg
incidence and severity levels were too low to draw any meaningful conclusions.
Fusarium
levels were also lower where fungicide was applied.
Fusarium severity was rated based on percentage of the head infected.
The average plot severity for all of the treated plots of wheat and
barley was 3.6 and 6.4%, respectively, compared with 5. 9 and 8.4% in the
respective untreated plots. The
average disease incidence for the treated wheat and barley plots was 35 and 59%,
respectively, compared with 40 and 64% in the respective untreated plots.
Low disease levels were probably a result of crop and cultivation
history. The field was in durum the
last two years with small grains prior to 1998.
The continuous small grain rotation resulted in very low SSR inoculum.
In addition, the field was deep plowed for the past two years, burying
any inoculum present for either SSR or Fusarium head blight.
HIGHLIGHTS
•No observations on crop
rotation could be made in 2000 because entire plot area was durum in 1999.
•Low levels of disease in
2000 were probably a result of deep tillage and continuous small grains in
cropping history.
•Fungicide-treated canola
had less incidence and severity of sclerotinia stem rot.
•Fungicide-treated wheat
and barley had less incidence and severity of Fusarium head blight.
ACKNOWLEDGEMENTS
This research was made
possible by funding from: Northern
Canola Growers Association, North Dakota Oilseed Council, and Agricultural
Utilization Research Institute.
LITERATURE
CITED
Morrall, R. A. A. and J. R.
Thomson. 1991. Petal test manual for Sclerotinia in canola. University of
Saskatchewan, Saskatoon, SK. 25pp.
Steadman, J. R., J. Marcinkowska and S. Rutledge. 1994. A semi-selective medium for isolation of Sclerotinia sclerotiorum. Can. J. Plant Pathology 16:68-70.