If severe, the fungus also attacks stems and fruit. Affected leaves may turn yellow, then drop, leaving the fruit exposed to sunburn. Sanitation is the best control. Remove all diseased plant tissue on the ground, as the fungus overwinters on leaf debris. Do not plant tomatoes in the same place next year.
Space plants farther apart to improve air circulation.
Avoid overhead irrigation. If the infestation is heavy, sulfur dust may help protect new leaves from infection. Septoria leaf spot is less common in Colorado than early blight. It, too, is a fungal disease. Characteristic symptoms are white or gray spots on leaves, surrounded by a black or brown margin.