Black Leg of Rapeseed
The severity of symptoms vary widely depending on the crop or variety in question, the pathogen itself and the prevailing environmental conditions. In any case, main symptoms are visible on leaves and stems. Leaf lesions include both circular, pale grayish lesions dotted with black specks, and dark necrotic lesions. A yellow discoloration of the leaf vein or of whole patches surrounding the lesions is also common (chlorotic halo). Stems also show grayish lesion that can range from small, oblong, brownish spots to cankers that girdle the whole stem. Blackish specks can also be observed on them. As they grow, the cankers girdle the stem and weaken it, leading to early ripening, lodging and plant death. Pods may show symptoms in the form of brown lesions with a black margin, resulting in premature ripening and seed infection.
Blackleg of rapeseed (also known as Phoma stem canker) is actually caused by two species of fungi, namely Leptosphaeria maculans and L. biglobosa. They overwinter in seeds, or in stubbles and crop debris that remain in the field. They begin to produce spores during the onset of warm and humid weather in spring. These spores are dispersed by the wind or splashing rain to healthy plant parts, mainly lower leaves and the base of the stem. The germination of the spore and the growth of the fungus on the plant tissues trigger the appearance of symptoms. If the cotyledons are infected, seedlings may die early in the season (damping-off). The fungus spreads from the young leaves to the stem, where it grows forming cankers at the junction between petiole and stem or around the crown. This restricts water and nutrient transport to through the stem, and death and lodging may ensue. It is an important disease on rapeseed and other crops of the Brassica family (canola, turnip, broccoli, Brussels sprouts, cabbage).
No biological control measures seem to be available to fight these disease. Please contact us if you know of any.
Always consider an integrated approach with preventive measures together with biological treat ments if available. Fungicides have very little effect once the fungus has reached the stem and treatment are only justified in fields where high yields are expected. Prothioconazole can be used as foliar spray. An active seed treatment with prochloraz added to thiram can reduce seedling infections caused by seed-borne Phoma infection.
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