What is turning canola grey/black?
According to AAFC plant pathologist, Dr. Kelly Turkington:
“Sooty moulds are a group of saprophytic fungi that grow on normally mature or prematurely ripened plants, turning them from a nice harvest golden brown colour to a dusty dark grey/black colour due to superficial growth of the sooty mould fungi. The main species that hitchhike onto and infect dead or damaged tissue are Alternaria spp. (not the pathogenic ones!) and Cladosporium spp. They typically occur if there are wet conditions post maturity that delay the timely harvesting of crops, or on prematurely ripened plant tissues killed by abiotic stresses (e.g. hail, flooding, drought, high temps) or biotic factors (root rots, insect damage, etc.) in moist conditions. Normally sooty moulds are not an issue in terms of yield loss, but instead may lead to excessive weathering and discolouration of plants and harvested crop.
Alternaria blackspot (pathogenic species of Alternaria) normally results in leaf lesions and then eventually blackish flecks/lesions on the pod tissues. It is very evident before plants start to turn colour and mature. The unaffected parts of the pods are still green, and you don’t see the general dusty grey/black that is typical of sooty mould. Alternaria blackspot typically affects the rarely grown Polish canola (B. rapa) more than Argentine canola (B. napus). Sulphur deficiency can contribute to a substantial increase in Alternaria blackspot.”
Some fungicide labels indicate Alternaria control in canola but are most likely mentioned as a side benefit from late or second pass sclerotinia fungicide applications. Like sclerotinia, by the time a grower identifies Alternaria infection that is severe enough to warrant fungicide, the spray decision is too late. With no or limited impact on yield, a targeted preventative fungicide application for Alternaria is generally economically impractical. Always read and follow label directions, including preharvest intervals. There are no effective management tools for sooty moulds.
