Treat your own seed with Thiram. Control outbreaks of the disease with a fungicide, such as chlorothalonil or Maneb. An organic approach would be to simply remove and destroy infected plants to prevent spread of the disease. Also, avoid working the garden after watering to reduce the chance of spreading diseases. Water early in the morning to give the plants time to dry off.
Early blight is a very common disease of both potato and tomato.
It causes leaf spots and tuber blight on potato, and leaf spots, fruit rot and stem lesions on tomato. The disease can occur over a wide range of climatic conditions and can be very destructive if left uncontrolled, often resulting in complete defoliation of plants. In contrast to the name, it rarely develops early, but usually appears on mature foliage. As the spots enlarge, concentric rings may form as a result of irregular growth patterns by the organism in the leaf tissue. There is often a narrow, yellow halo around each spot and lesions are usually bordered by veins.