This fungus disease attacks tomatoes and potatoes over most of North America. Plants under stress or with a heavy load of fruit are most susceptible. Dark brown spots with concentric rings in them form on older leaves first. Infected leaves turn yellow and die. Potato tubers are covered with brown, corky spots. Tomato fruits may sometimes be infected; a black, sunken, leathery spot forms at the stem end. Warm, moist conditions encourage disease development. The fungus overwinters in plant residues in the soil.
Plant in well-drained soil where air circulation is good. Rotate crops and destroy any volunteer potato or tomato plants. Don't wet foliage when watering. Amend soil with compost, and fertilize plants judiciously to maintain plant vigor. Use certified disease-free seed potatoes and tomato transplants. Tomato plants with early blight slowly lose their leaves, but unless the infection is severe, you can usually harvest mature tomatoes. Use a copper-based fungicide as directed on the product label at first sign of the disease.
Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org