Mycosphaerella brassicicola (anamorph: Astromella brassicae)
Worldwide in cool, moist climates
Lesions manifest as water-soaked areas surrounded by chlorotic haloes, which are visible on both leaf surfaces and stems. On leaves, lesions can expand in size to 2.5cm (1 in.) in diameter. Fruiting bodies often form concentric rings within lesions. Lesions may coalesce, giving leaves a yellow, tattered appearance. On stems, lesions are often rectangular to oval. The disease may also cause a storage rot of cabbage, leaving it shriveled and leathery.
Conditions for Disease Development
Infected debris serves as the primary inoculum source. The fungal spores (ascospores) are spread by wind, and infection occurs through the stomata. Cool [15-21°C (59- 70°F)], moist weather generally favors disease development.
Remove and destroy crop refuse. Locate seed beds away from existing cruciferous crops. Apply preventive fungicide sprays to help control this disease.