Answer: I don't know for certain without seeing the plant but it sounds like Early blight (Alternaria leaf spot). This disease is caused by the fungus Alternaria solani. Symptoms become prevalent during the hotter months. This disease produces brown to black, target-like spots on older leaves. 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. If the infestation is heavy, sulfur dust may help protect new leaves from infection. 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 to avoid spread of the disease.
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