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Symptoms and Signs
The first obvious symptom of blackleg of oilseed rape (rapeseed) (Brassica napus, B. rapa, and some other Brassica spp.) caused by the fungus Leptosphaeria maculans, is the appearance of gray-green to ash-gray lesions on the lower leaves (Figure 2). The presence of small, black pycnidia at the edge or scattered across the blackleg lesions distinguishes them from lesions caused by another common foliar pathogen, Alternaria brassicae (Figure 3). Alternaria lesions have no pycnidia and typically contain concentric rings. Tissue in a lesion may dry, crack, and fall out, making identification difficult (Figure 4). Blackleg lesions from multiple infections may coalesce (Figure 5). The lesions often expand down leaf veins towards the base of the leaf. In severe epidemics, lesions also can be found on the stems and pods of oilseed rape plants.
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Basal stem lesions are the most damaging. When these occur in the seedling or rosette phases of growth, symptoms resemble damping-off or cut-worm damage (Figure 6). In older plants, the more typical canker symptom (Figures 7 and 8) leads to premature ripening or lodging (falling over) of the crop (Figures 9, 10, and 11).
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Copyright © 2000
by The American Phytopathological Society