Serious cabbage diseases can be introduced to a clean garden on diseased transplants. Therefore, it's important that a gardener plant disease-free transplants. Blackrot and wirestem cannot be controlled with fungicides or other plant disease chemicals. This disease often shows up following several days of rainy, overcast weather, when temperatures are on the cool side. Young plants show a white mildew growth on the underside of the leaves.
Infected leaves turn yellow and quickly drop from plants.
Since the disease may destroy most of the foliage on young plants in short order, it's important to spot this disease and start a fungicide spray program before downy mildew destroys the planting. At the same time you are checking plants for downy mildew, look for early symptoms of Alternaria leafspot. This fungus disease is also favored by wet weather. Left uncontrolled, Alternaria leafspot destroys foliage and the developing cabbage head. The first symptom of this disease is a small dark spot on the oldest transplant leaves.