Tomatoes are a favorite summer vegetable for many gardeners. In Missouri, many disease problems can ruin the chance of a crop, shorten the harvest season and reduce yield, or reduce the quality of the fruit. In fact, the health of the plant can even have an impact on the flavor of the tomato. One of the most frequent diseases seen during the summer is Septoria leaf spot. This disease infects the lower leaves first and gradually works its way up the plant. The first symptom usually appears after the first fruit sets with small (1/8 to 1/4 inch) circular dark-brown borders and gray centers. By looking at the lesion with a hand lens, you can see little, pepper-like fruiting bodies, which produce spores. The disease is spread when rainfall or overhead irrigation splashes the spores onto healthy tissue. As more spots develop, entire leaves will turn brown and dry up. Septoria leaf spot can rapidly progress over the entire plant during warm and rainy periods.
One of the most important things you can do is begin with strong, healthy seedlings that show no signs of disease. Vigorous, modern tomato varieties often develop fewer symptoms than older heirloom varieties, although very little resistance exists in common tomato varieties. Wet foliage is required for this disease to infect tomato leaves. Spacing and staking plants upright will promote good air circulation and rapid drying of foliage. Overhead watering should be avoided; instead water at ground level with a soaker hose or direct the nozzle below the foliage. If this disease occurs this season, you cannot cure plants once they have become infected, but Septoria-infected plants will often produce well anyhow, especially when disease develops later in the season. The Septoria spot fungus can survive between crops on infected tomato debris, so remove and destroy debris soon after the plants stop producing. If Septoria leaf spot are severe year after year and limit production, fungicide applications that contain chlorothalonil, maneb, mancozeb or copper applied every 7 to 10 days can help in management. Fungicides should be used in addition to the other management tactics, and according to the label.
REVISED: December 2, 2011