Alternaria leaf spot

Alternaria brassicicola

The genus Alternaria is comprised of many common saprophytic (derive nutrients from dead and/or decaying organic matter) and plant pathogenic species.
Alternaria brassicicola causes black spot disease (also called dark leaf spot) on virtually every important cultivated Brassica species including broccoli, cabbage, canola, and mustard. It is of worldwide economic importance resulting occasionally in 20-50% yield reductions in crops such as canola, mustard or rape. Like other diseases caused by Alternaria species, black spot appears on the leaves as necrotic lesions, which are often described as black and sooty with chlorotic yellow halos surrounding the lesion sites.
Symptoms. The two Alternaria species, A. brassicae and A. brassicicola, cause similar symptoms; small, dark specks first develop on leaves and later enlarge into circular, tan spots (0.25-0.5 inch in diameter). The spots caused by A. brassicicola tend to be darker than those caused by A. brassicae. If conditions are favorable, dark green spores of the pathogen will grow on the spots. Such growth causes the spots to have concentric rings in them. Old leafspots become papery in texture and may tear. When the dry tissue falls out, a shothole effect results.

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