Impact
of preceding crops on incidence and severity of disease in canola (2004)
A
four-year rotation study was initiated in 2000 to determine the impact of
preceding crops on disease incidence and severity in canola.
Six rotations were evaluated and every phase of the rotation is present
every year in a randomized complete block design replicated four times.
The rotations consist of canola every one, two, three, or four years
preceded by either canola, flax, or wheat.
Half of each canola plot was treated with fungicide to prevent
Sclerotinia stem rot (SSR). Plots
were evaluated for SSR risk; SSR and blackleg incidence and severity; and yield
and test weight.
Sclerotinia
ascospore levels detected by petal and Steadman tests indicated very low disease
risk in 2000. Low ascospore levels
in 2000 were likely due to lack of inoculum in the area as the field history was
cereal grains for twenty years prior to the initiation of the study.
In
2001, increased inoculum and favorable environment resulted in moderate disease
risk, 65, and 100% incidence, detected by the early Petal and Steadman tests,
respectively. However, little
precipitation between the first and second evaluation caused a dramatic drop in
disease risk. Risk is considered low at 0 to 45%, moderate at 45 to 95%, and
high at 90 to 100% incidence on the petal test (Morrall and Thompson 1991).
In
2002 and 2003, the petal test indicated very low disease risk at both evaluation
dates. However, the Steadman test
in 2002 detected a higher level of ascospores than the petal test, possibly due
to the microclimate within the crop canopy being more favorable for sporulation.
In 2004, petal tests indicated moderate SSR disease risk, 56 % incidence;
and weather conditions have been more favorable for plant infection than in
previous years of the study.
To
date, general observations on disease risk indicate it is more dependent on
environment than rotation.
SSR
disease incidence on standing canola plants adjacent to canola swaths was too
low to detect any significant differences throughout the study.
SSR disease incidence peaked at 5% in canola on canola rotations in 2001,
where risk test indicated the highest level of ascospores, but canola yield was
not adversely affected. In 2000
through 2004, there was little risk or incidence of SSR, regardless of rotation
or fungicide treatment, in this study.
Blackleg
incidence has gradually increased each year, until 2004.
There was very little blackleg detected in 2000, the first year of the
study. In 2001, blackleg incidence
was up to 8% in canola on canola rotations.
In 2002, the third year of the study, blackleg incidence was 37% in
canola preceded by two years of canola, 24% in canola on canola, and less than
10% in first year canola or canola preceded by wheat preceded by canola (canola
every other year). Although
blackleg incidence in canola every other year was slightly higher than in first
year canola, it was not significantly different.
In 2003, canola once in four years and canola once in three years had
similar blackleg incidence, 22%. Blackleg
incidence was higher in canola preceded by three consecutive years of canola,
45%, which was similar to canola preceded by canola, and canola preceded by
wheat preceded by canola (canola every other year).
Blackleg severity did not increase with the occurrence of canola in the
rotation and yield was not affected by blackleg incidence in 2002 or 2003.
The lack of yield response to higher blackleg incidence is likely due to
the blackleg resistance of the canola variety planted, as well as
below‑normal precipitation and high temperatures during flowering in 2002
and 2003. In fact, overall canola
yields were down in 2002 and 2003 compared to 2000 and 2001.
However, in 2004, blackleg incidence was variable, and not related to
rotation. This is likely due to heavy rain and hail in early June which
damaged young canola plants that had shown symptoms of blackleg.
The canola had recovered, but the blackleg lesions did not reoccur.
Literature
Cited
Morrall, R. A. A. and J. R.
Thomson. 1991. Petal test manual for Sclerotinia in canola. University of
Saskatchewan, Saskatoon, SK 25pp.
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