Control is difficult and must be accomplished before damage occurs. Cultural practices such as turning under old plant debris in the fall hastens breakdown of the crop residue and reduces disease carryover. Rotation of one year for black rot and three years for blackleg will help avoid these diseases in the garden. Infected plants wilt several weeks after transplant and lower leaves turn yellow. Alternaria leaf spot, a foliage disease, produces brown, round spots on the leaves and can cause small brown to black spots on the heads of cauliflower.
Downy mildew causes purplish, irregular spots on the leaves of infected plants.
A white mold can be seen growing on the underside of infected leaves during wet weather. The fungus can infect the heads of broccoli and cauliflower and cause loss of quality. To control leaf spot disease, plant disease-free transplants and spray with a fungicide when leaf spots are first seen or during wet periods in the fall. Bacterial soft rot on the heads of broccoli and cauliflower can be seen during periods of warm wet weather in the early fall. Large areas of the heads turn brown or black and are soft and mushy.