Black spot

Alternaria raphani

Alternaria brassicae, Alternaria brassicicola, Alternaria raphani - These pathogens affect most cruciferous crops, including broccoli and cauliflower (Brassica oleracea L. var. botrytis L.), field mustard and turnip (B. rapa L. (synonym: B. campestris L.), leaf or Chinese mustard (B. juncea), Chinese or celery cabbage (B. pekinensis), cabbage (B. oleracea var. capitata), rape (B. campestris), and radish (Raphanus sativus).
A. brassicae and A. brassicicola can affect host species at all stages of growth, including seeds. On seedlings symptoms include dark stem lesions immediately after germination, that can result in damping-off, or stunted seedlings. A. raphani produces black stripes or dark brown, sharp-edged lesions on the hypocotyl of the seedling. It grows in the vascular system and rapidly infects the entire seedling. When older plants become infected, Alternaria symptoms often occur on the older leaves, since they are closer to the soil and are more readily infected as a consequence of rain splash or wind blown rain. Late infection, or infection of older leaves, does not characteristically reduce yiedls, and can be controlled through intensive removal of infected leaves. Fruit-bearing branches and seed pods show dark or blackened spots that result in yield loss due to premature pod ripening and shedding of the seeds. Infection can also occur on the fruit, before or after harvest. A common symptom of broccoli and cauliflower infection is a browning that occurs on the head.
All three pathogens are destructive diseases for seed growers. The pathogens can shrivel seeds within the pods or kill the pod stalks before seed formation. They may also be a means by which bacterial soft rot enters the stem, which may lead to plant death. In addition to destruction of a seed crop, the pathogens can live within the seed, spread the disease to other fields, and cause a loss of seedlings.

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