The disease Alternaria leaf blight is caused by two related plant pathogens, A. brassicae and A. brassicicola. The disease complex is known to affect a wide variety of crucifer plants at all stages of growth. The two species of Alternaria pathogens cause very similar symptoms and may occur at the same time on a host plant. The disease is enhanced by conditions of warm, moist weather with optimal infection temperature ranging from 25-30oC and moisture on the leaf surface.
Leaf blight lesions vary in size from pinpoints up to 6 cm in diameter. Lesions often present themselves as yellow spots that enlarge up to 1.5 cm in diameter with typical darker concentric rings. The lesions often occur on leaf blades and petioles. If lesions grow together or the infection persists on petioles, the leaves can die. Lesions contain many spores which are spread by wind, rain or on equipment and people.
Seed treatments are highly recommended for the prevention of infection; however, if seed treatment is not used, then a chemical fungicide should be used at the first sign of infection, or if conditions are optimal for Alternaria infection.