Brassica Alternaria Leaf Spot is a fungal disease of brassica crops caused by Alternaria brassicicola. Symptoms begin as small, black lesions on leaves that enlarge to form target-like concentric rings of necrotic tissue surrounded by a yellow halo. When humidity is high, these lesions produce black spores on the leaf surface. A. brassicicola survives on infected seed, as well as infected soil-bound plant debris and host brassica weeds. It spreads primarily via windborne spores, as well as rain and irrigation splash and some insects.
Favorable conditions include moderate to high temperatures and high humidity. Cultural management involves using disease-free seed, destroying infected debris, maintaining a 3-year rotation between brassica crops (including broccoli, cabbage, collards, Brussels sprouts), and managing both host brassica weeds and insect vectors. Well-timed fungicide application can also control Alternaria leaf spot.