White Leaf Spot and Gray Stem

Table of contents

    White Leaf Spot and Gray Stem

    This disease is caused by the fungus Pseudocercosporella capsellae. White leaf spot and gray 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

    Figure 35. Gray Stem

    Photo by Beth Hoar

    White leaf spot appears in the summer. Severe leaf spotting can result in premature leaf loss. As the crop ripens, large purple to grey-speckled stem and pod lesions develop. 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.

    Disease Cycle

    The 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, white to buff-coloured spots develop on lower leaves. These lesions produce wind-borne spores that cause 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 help reduce infection. Good crop production practices that reduce stress due to weed competition and nutrient deficiency help delay disease development.