Alternaria blackspot has caused extensive yield losses of canola in recent years in western Canada. Yield losses were higher in Polish than with the more resistant Argentine canola. In addition to yield loss and reduced seed germination, the disease also increases green seed counts and dockage levels . The severity of the disease depends largely on environmental conditions during podding and maturity. Frequent rains or dews and temperatures near 20°C favor epidemics.
Three species of Alternaria fungi are commonly present on canola in western Canada: A. brassicae, A. raphani and A. alternata. A. alternata is a much weaker pathogen than the other two species. It is a common saprophyte on dead and dying material and is noticeable as a black mold on mature plants during wet harvests.
Appearance: Seedlings infected directly from infested seed may exhibit damping off or wirestem disease (seedlings curl up). Leaf spots are initially small and brown or black. The spots enlarge and are usually gray or black surrounded by a yellow halo. The most conspicuous symptoms are black spots on stems and pods as plants approach maturity. Infected seeds are often shriveled and may have a black or gray discoloration.
Disease Cycle: Blackspot fungi over-winter on infected plant residue and on/in seed. Several weeds of the mustard family are hosts of Alternarias. Alternaria spores can be dispersed by wind or rain-splash.
Canola plants vary in their susceptibility to Alternaria attack throughout their development. Aging plants are more susceptible than young and intermediate-aged plants. Therefore, Alternaria Blackspot epidemics intensify at flowering and reach their maximum intensity in senescing plants.
a) Use well-cleaned seed that has been tested for Alternaria. Seed that contains high levels of A. brassicae or A. raphani will probably have poor germination. Chemical seed treatments are effective in reducing seed-borne Alternaria but the overall vigor of the seed may be reduced by Alternaria infection.
b) Type of canola – Growers can lessen the risk of serious losses by sowing B. napus rather than B. rapa canola. B. napus varieties have more leaf wax, which provides more tolerance to black spot disease. However, this may not be a practical option for growers in some areas because of the later maturity of B. napus canola varieties.
c) Early swathing of heavily infected fields might reduce losses from the disease.
d) Aerial fungicide applied at 95% flowering provides economical control of Alternaria.