Symptoms: Common on tomato and potato plants, early blight is caused by the fungus Alternaria solani, and occurs throughout the United States. It first appears on the lower, older leaves as small brown spots with concentric rings that form a “bull’s eye” pattern. As the disease matures, it spreads outward on the leaf surface causing it to turn yellow, wither and die. Eventually the stem, fruit and upper portion of the plant will become infected. Crops can be severely damaged.
Early blight overwinters on infected plant tissue and is spread by splashing rain, irrigation, insects and garden tools. The disease is also carried on tomato seeds and in potato tubers. In spite of its name, early blight can occur any time throughout the growing season. High temperatures (80-85 degrees F.) and wet, humid conditions promote its rapid spread. In many cases, poorly nourished or stressed plants are attacked.
Control: To avoid early blight, prune or stake plants to improve air circulation. Make sure to disinfect your pruning shears (one part bleach to 4 parts water) after each cut. Keep the soil under plants clean and free of garden debris. Add a layer of mulch to prevent the spores from splashing back up onto vegetation. Water in the early morning hours (avoid overhead irrigation) to give the plants time to dry out during the day. Rotate susceptible crops to other areas of the garden each year. If symptoms of early blight are present, begin applying a copper based fungicide weekly until harvest. Organic fungicides will not kill the disease, but prevent the fungal spores from germinating.