Both early and late blight are caused by fungal infections that can quickly destroy foliage and fruit. The fungus Alternaria solani causes early blight, characterized by dark lesions with concentric circles that cause foliage to dry a drop from the plant. Late blight, caused by the Phytophthora infestans, causes brown lesions on foliage, often accompanied with a white mold on the undersides of leaves. Fruit develops dark rough lesions that may progress to a moist rot of the entire fruit. Once infected the best remedy is to destroy affected plants, however, preventative measures greatly reduce the incidence of both early and late blight.
Provide good air circulation by spacing tomato plants at least 2 feet apart. Large indeterminate varieties may require 3 feet between plants. Pruning to keep plants in shape and reduce overgrown foliage increases aeration and reduces the risk of disease.
Plant tomato plants in rows facing the prevailing winds in your area. This increases air circulation and assists in drying wet foliage after rains.
Water tomatoes early in the day so foliage has time to dry before nightfall. If you must water in the evening, do so early to allow plants to dry before sunset.
Remove any infected plant material and destroy. Double bag plant debris and tie securely before disposing of it in the trash. Do not add infected plant material to the compost bin as the spores may contaminate compost.
Rotate crops to reduce the risk of blight. These soil-borne fungi can survive in the soil, but with the exception of potatoes and some weeds, but do not affect other crops.
Begin treatment with fungicide before symptoms occur and maintain a regular schedule. Check that the fungicide is designed for blight on tomatoes and follow the recommended application rate. Once symptoms appear it is generally too late to save infected plants. Applying fungicide kills spore before they can infect your plants.