Septoria leaf spot, caused by the fungus Septoria lycopersici, is the most common foliar disease of tomatoes in Iowa. It first appears as small, water-soaked spots that soon become circular spots about 1 ⁄8 inch in diameter (Figure 1).
The lesions gradually develop grayish white centers with dark edges. The light-colored centers of these spots are the most distinctive symptom of Septoria leaf spot. When conditions are favorable, fungal fruiting bodies appear as tiny black specks in the centers of the spots. Spores are spread to new leaves by splashing rain. Heavily infected leaves turn yellow, wither, and eventually fall off. Lower leaves are infected first, and the disease progresses upward if rainy weather persists.
Defoliation can be severe after periods of prolonged warm, wet weather. Infection can occur at any stage of plant development but appears most frequently after plants have begun to set fruit. The fungus survives the winter in tomato debris.
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