At this time of year, symptoms of Septoria leaf spot are quite evident on tomato leaves. This disease usually appears on lower mature leaves, causing them to turn yellow then brown and wither. Diseased leaves show small circular spots (lesions) with black or brown margins. The center of these lesions eventually turns gray, with small black fungal fruiting structures (pycnidia) scattered throughout. Lesions may also occur on stems and petioles, but tend to be more elongated.

Septoria leaf spot is caused by the fungus Septoria lycopersici. High relative humidity and temperatures between 68-77 F are favorable conditions for infection and development of this disease.

Listed below are measures that can help control the disease:

1. Plant disease-free transplants.
2. Space plant appropriately for good aeration.
3. Stake plants and mulch to reduce contact with the soil.
4. Do not water plants from above. The spores on infected leaves are transferred by splashing water.
5. Remove any diseased leaves to slow down infection.
6. A fungicide spray program can be used to help control the disease. Check your garden center or available fungicide products.
7. Do not work with plants during wet conditions so as not to transfer spores to other plants.
8. Control susceptible weeds such as nightshade, jimson weed, and horse nettle to reduce the amount of fungus that may overwinter.
9. Clean infected garden debris in the fall and till the ground. It would also be beneficial to retill again in the spring.
10. Rotate susceptible crops. Do not grow tomatoes, peppers, or eggplants in that area for three years.

This article originally appeared in the August 11, 2000 issue, p. 105.