This plant disease is caused by a fungus (Alternaria solani) that attacks both vines and tubers. It is most severe toward the end of the growing season when the vines approach maturity and after tubers are formed. Many leaves may be killed. The potato yield is reduced, but the plant seldom dies. Tubers are frequently infected through wounds inflicted during harvest. Early blight is favored by moisture and temperatures in the range of 75° to 85°F. The fungal spores spend the winter in plant debris left in the garden. Infected tubers are inedible.
Spray plants with a fungicide containing chlorothalonil as soon as leaf spotting occurs. Repeat the treatment every 7 to 10 days until the leaves die back naturally. Clean up and destroy plant debris after harvest. Do not store infected tubers. Next year, begin spraying the plants when they are 6 inches tall, and continue at intervals of 7 to 10 days until the tops die back. Avoid overhead watering by using drip or furrow irrigation. Use seed potatoes certified by state departments of agriculture to be free of diseases.
Plant Care for Potatoes