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Black spot occurs wherever canola-rapeseed is grown and can
affect all stages of growth and all plant parts except roots. It is most often
seen on developing pods where it causes direct losses beneath each lesion,
usually to 5 percent of the crop, but occasionally much higher. Serious losses
have occurred in both wet and dry summers.
Symptoms:
Infected seed may rot in the ground, or may produce infected
seedlings with dark spots on the cotyledons. Leaf spots usually appear in early
summer on lower leaves as circular, pale to brownish grey lesions with darker
concentric lines. Leaf spots may vary in size and colour, depending on
environmental conditions. They are sometimes almost black in colour. Leaf spots
produce spores which subsequently infect pods and stems, forming circular, black
or dark brown spots which may elongate into irregular blackish mottled elongate
lesions. Pod lesions are frequently sunken, and the seeds beneath them are
shrivelled. Infected pods may ripen prematurely, and shatter before the crop is
mature resulting in yield loss.
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Alternaria black spot on canola pods and stems |
Life Cycle:
Alternaria fungi survive the winter on infected crop debris,
seed, and on mustard family weeds. It is particularly prevalent on stinkweed.
Wind-blown spores are produced on crop residue in the spring, which are able to
infect canola leaves during favourable (moist) environmental conditions. New
spores are produced on diseased plants, which may cause more leaf, stem and pod
infections. Seeds may become infected following development of black spot on the
pods.
Control:
Cultural: Use disease-free seed
of high germinability. If seed from an infected crop must be used, the seed
should be cleaned thoroughly to remove shrunken, severely infected seed before
planting. Rotate with non-cruciferous crops for at least three years. Control
volunteers and cruciferous weeds (e.g. shepherd's-purse, wild mustard), and
especially stinkweed during the rotation. Incorporate infected stubble into soil
thoroughly. Swath badly infected crops on time to prevent serious losses from
shattering and seed shrinkage.
Resistant Cultivars: B. rapa
(Polish type) cultivars are more susceptible than B. napus (Argentine
type) cultivars. B. juncea (brown and oriental mustard) is also
susceptible.
Chemical: The fungicides Quadris
(azoxystrobin) and Lance (boscalid) are registered for control of Alternaria
black spot on canola. Rovral Flo (iprodione) is registered for suppression of
black spot. Good coverage of plants is essential. Follow label directions.
- Quadris: Apply once as a broadcast foliar spray at the
early pod stage (90% petal fall) of canola. Do not apply within 30 days of
harvest.
- Lance: Apply at late flowering to early green pod. Do not
make more than 2 applications per season. Do not apply within 21 days of
harvest.
- Rovral Flo: Apply as a single spray at early green pod,
or as split application: the first at full bloom, followed by a second
application at early green pod stage. Do not make more than 2 applications
per season. Do not apply within 38 days of harvest.
Using fungicide-treated seed may help to increase stand
establishment when infected seed is planted. Seed treatments registered for
control of seed-borne Alternaria include: Foundation Lite, Gaucho CS FL, Helix,
Helix XTra, Prosper FL and Vitavax rs Fungicide.
Updated July 2007
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