Young seedlings can be killed by black rot. Established plants quickly lose their quality, and should be pulled up and composted. If left in the garden, windblown rain can spread black rot to nearby cabbage family plants. Produce harvested from infected plants often rots in storage.
The bacteria that causes black rot can be carried in seed, so always purchase disease-free seeds, and do not save seeds from infected plants. When buying seedlings, choose plants with no suspicious blemishes on their leaves. Choose resistant varieties when growing susceptible crops in warm climates. A good rotation to prevent the buildup of black rot bacteria is to plant cabbage family crops after legumes or tomatoes. Use mulch to keep soil from splashing onto plants in heavy rains.
Symptoms of black rot often appear more than a week after the infection is established, when little can be done to control it. Promptly pull up plants that show symptoms and dispose of them in an active compost pile, so they decompose completely by the following summer. Diseased plant material that is simply dug into the soil can persist for up to two years.