Early blight, caused by the fungus Alternaria solani, is also known as Alternaria leaf spot or target spot. Like Septoria leaf spot, early blight is common in Iowa tomato plantings, and the two diseases may attack the same plants. Premature loss of lower leaves is the most obvious symptom of the disease.
Brown to black spots, 1 ⁄4 to 1 ⁄2 inch in diameter with dark edges, appear on lower leaves. Spots frequently merge, forming irregular blotches. Dark, concentric rings often appear in leaf spots, resulting in the “target” appearance suggested by the common name. Leaves turn yellow and dry up when only a few spots are present. The fungus occasionally attacks fruit at the stem end, causing large, sunken areas with concentric rings and a black, velvety appearance.
Warm, wet weather favors rapid spread of early blight. A. solani also can infect potato. Like Septoria leaf spot, early blight can infect plants at any stage during the growing season but usually progresses most rapidly after plants have set fruit.
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