Leafy greens are not appealing to eat when covered with spots. When almost-perfect heads of broccoli or cauliflower become infected, dark sections of rot are often invaded by other microorganisms, making a bad situation worse.
Choose resistant varieties when they are available. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying after rains. Use mulch to keep soil from splashing onto plants in heavy rains. Avoid using sprinklers or other overhead irrigation methods after cabbage, broccoli or cauliflower heads have formed.
Clip off and compost badly affected leaves. With the return of dry weather, leafy greens will usually produce a new crop of sound leaves.