Alternaria leaf spot can weaken plants, but it will not kill them. However, when already infected plants receive more than twelve hours of constant warm rain, spores can be distributed so extensively that the plants cannot recover.
Alternaria leaf spot can be carried on seed, so always purchase disease-free seeds, and do not save seeds from infected plants. 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. At the end of the season, pull up and compost plant material so it will be completely rotted by the following summer.
Break off badly affected leaves and compost them. Removing older leaves that show alternaria spots will greatly reduce the number of spores present, and improve air circulation around the plants at the same time.