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Septoria Leaf Spot of Tomatoes

by Richard M. Riedel, Sally A. Miller, Randall C. Rowe    

Septoria leaf spot is a disease of the foliage and stems. It does not affect fruit directly. The disease causes rapid defoliation when weather is warm and moist.

Symptoms

Although the symptoms may appear on the leaves and stems at any stage of plant development, they usually become evident after plants have begun to set fruit in mid-July. Initially the fungus causes small, water-soaked, roughly circular spots scattered randomly over the leaf. These spots enlarge to become l/l6 to l/8 inch in diameter with dark margins and tan centers. Small, dark, pimple-like fruiting bodies in which spores of the fungus are produced can be seen in the center of the lesions. Older leaves near the ground usually show symptoms first. Symptoms appear rapidly on younger foliage in rainy weather. When leaves are heavily infected, they drop prematurely. Exposed fruits are subject to sunscald. The fungus which causes this disease can grow on weeds such as Jimson weed, horse nettle and nightshade.

Causal Organism

Septoria lycopersici lives between tomato crops in the soil on infested debris of tomato and weeds. Spores formed on crop debris splash onto foliage and start the disease. Wind and rain spread spores produced in the dark bodies formed in leaf spots to adjacent uninfected leaves. The fungus is most active between 60 and 80 degrees F when rainfall is abundant.

Control:

1. Rotate out of tomatoes for 4 years.

2. Deep plow, preferably in the fall, to bury all plant refuse.

3. Grow tomato transplants in sterilized soil.

4. Control weeds, especially horse nettle, Jimson weed, and nightshade.

5. Use of a protectant fungicide may be necessary to adequately control Septoria Leaf Spot when conditions are favorable for disease development. For effective fungicides and spray schedules consult the Ohio Vegetable Production Guide (OSU Extension Bulletin 672).