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Causal Agent

The fungus, Alternaria solani 






Early Blight of TomatoDark brown cankers may develop on and girdle stems of seedlings at the soil line. Stem lesions on older plants usually remain confined to one side of the stem. Leaf symptoms appear on older leaves first and are characteristic of the disease. Infected leaves show circular spots (lesions) up to 1/2 inch in diameter that are brown and have a pattern of concentric rings imparting a "target-like" appearance to the spot. The leaf area surrounding these spots usually turns yellow. Infected leaves eventually turn brown and drop from the plant. Defoliation progresses upward from the lower plant. Sunken, dry lesions occur most frequently on the stem end of the fruit and also have a zonate or "target-like" appearance. The fungus survives in the soil by forming resistant spores in association with diseased tomato debris that are capable of persisting for one year and probably longer. Infection occurs rapidly under warm, humid conditions. Thousands of spores are produced in spots of infected leaves that are capable of causing more infections. Plants under stress from nitrogen deficiency, heavy fruit load, or other factors are most susceptible to the disease. 



Crop rotation with crops other than eggplant, potato, and pepper should be practiced to reduce and delay early blight development. Avoid prolonged wetting of leaves from irrigation or use drip irrigation. Maintain adequate but not excessive soil fertility. A spray program using a recommended fungicide beginning at fruit set and continuing on a 7-14 day schedule should be maintained where early blight problems are anticipated. Please contact your local county extension office for current information.

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