White leaf spot and grey stem

White leaf spot and grey stem are both canola diseases caused by the fungus Mycosphaerella capsellae (anamorph Pseudocercosporella capsellae). White leaf spot and grey stem are widespread throughout the canola growing areas of Western Canada. Although this disease can be found in most fields when the crop is ripening, it usually develops too late in the growing season to affect crop yields significantly.

Symptoms

Grey stem
Grey stem; Photo credit Beth Hoar

White leaf spot can first appear on leaves. These lesions frequently appear at leaf margins and are small grey-brown necrotic spots about one to two millimetres in diameter, which grow to a tan-coloured lesion five to 10 millimetres in size. The lesions can be confused with downy mildew. Severe leaf spotting can result in premature leaf loss. The grey stem symptoms appear later in the canola crop’s development in the form of large purple to grey-speckled stem and pod lesions. At harvest some plants may be completely discoloured and frequently the entire field turns a purple or grey colour. Disease symptoms may appear earlier in the season on crops under stress from lack of moisture, insufficient nitrogen or severe competition from weeds.

Grey stem lesions may be confused with blackleg lesions, but can be differentiated as stem lesions on the former are superficial and do not affect the pitch, while blackleg lesions are decayed and grey beneath.

Disease cycle

Grey stem
Grey stem diagnosed in a field in August

The disease-causing fungus overwinters as thick-walled mycelium on crop residue and produces wind-borne spores in the spring that infect canola plants. Following infection in early summer, lesions develop on lower leaves. These lesions produce wind-borne spores that re-infect other plant tissues, causing the rapid spread of the disease in the ripening crop. The disease is usually not seed-borne. The disease has a wide host range among cruciferous weeds, including shepherd’s purse, hare’s ear mustard and ball mustard.

Management

Crop rotation and control of volunteer canola and cruciferous weeds helps reduce infection. Good crop production practices that reduce crop stress due to weed competition and nutrient deficiency also help delay disease development.

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