Blackleg
Blackleg of canola, caused by the fungus Leptosphaeria maculans, can cause high
yield losses in susceptible cultivars. Two strains of the pathogen are present
in western Canada; a highly virulent and weakly virulent strain. The highly
virulent strain, a stronger pathogen, is now widespread in all three Prairie
Provinces. Most of the new canola cultivars now grown in western Canada have
improved blackleg resistance compared to the older cultivars, e.g. Westar.
Appearance: The most conspicuous symptoms are found on leaves and stems.
Leaf lesions are dirty white, round to irregular, and speckled with small black
dots (pycnidia). Stem lesions are white or gray and also contain numerous
pycnidia. The stem lesions often begin near the base of the scar remaining from
a fallen infected leaf. The lower portion of the stem becomes completely
girdled in severely infected plants. These plants ripen prematurely and produce
shriveled seed. The pods of infected plants have an increased tendency to
shatter.
The weakly virulent strain attacks plants as they near maturity. Lesions tend to
be shallow causing minimal damage. The highly virulent strain attacks the crop
earlier causing deep cankers that often completely sever the plant at the base
of the stem.
Disease Cycle: The blackleg fungus can over-winter in infected seed or on
residues and travel in the air to infect plants beginning at the seedling
stage. New infections produce pycnidia, from which spores exude under wet
conditions. These spores are spread by rain-splash, resulting in localized
spread of infection.
Disease Control:
a) Disease resistance - Canola cultivars rated as moderately resistant or
resistant should always be grown.
b) Crop rotation - Canola should no be grown more than once every four year on
the same field since infected plant residues are an important source of
infection. It is important o control volunteer canola and wild mustard to
prevent disease build-up.
c) Use disease-fee seed - Seed should be tested for the presence of the highly
virulent strain of the fungus. This is especially important for growers that
intend to plant canola on fields that do not have a history of blackleg in
order to prevent the introduction of the disease. Seed-borne infection can be
controlled by several chemical seed treatments.
d) Chemical control - the fungicide Tilt is registered as a foliar spray for
the control of blackleg. Or rates and application timing, contact your Novartis
representative.
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