Tomato Notes

As gardeners, we have all experienced occasional disappointments with our vegetables. For example, we choose a favorite tomato variety, coax the seed into germinating, water and fertilize to produce a beautiful transplant, and plant after danger of frost into our well-tilled and fertile garden. We mulch, stake and fertilize, carefully watching our plant grow and form potentially great tomatoes. Then, seemingly from nowhere, tragedy strikes. The plants start showing signs of disease, the leaves yellow and the once vigorous vine wilts. Why?

Unfortunately, wilts are part of the challenge of successful tomato production, but they do not have to happen. Let’s consider some probable causes of tomato wilting.

Verticillium wilt: Verticillium wilt is caused by a soilborne fungus. Most newer varieties of tomatoes (for example, ‘Better Boy,’ ‘Beef Master,’ ‘Celebrity,’ etc.) have resistance/tolerance to verticillium wilt. However, the older, "heirloom" varieties do not.

Verticillium wilt on tomatoes

In most seed catalogs, verticillium wilt resistance is noted by a "V" usually placed after the varietal name. The first symptom of this disease is midday wilting, then recovery at night. The leaves turn yellow and show a "v-shaped" lesion on the lowest leaves. Verticillium wilt is more prevalent in cooler weather, and symptoms are more pronounced when the plant has a heavy fruit load. The practical way to control this disease is to choose a good, disease-resistant variety and rotate three to four years between tomatoes and related crops such as eggplant, pepper and potatoes.

Fusarium wilt: Fusarium wilt is more prevalent in warmer weather. Like verticillium wilt, it is caused by a soilborne fungus that

Fusarium wilt on tomatoes

attacks the plant through the root system. A quick test to determine if your tomato may have fusarium wilt is to cut into the lower stem near the ground. If the center of the stem exhibits some discoloration, it is likely fusarium. The best management strategy for this disease is choosing a resistant variety. Most of the newer tomato varieties will have some resistance or tolerance to fusarium. An "F" will follow the variety name indicating resistance. Also, follow a rotation similar to the recommendation for verticillium wilt.

Nematodes: Nematodes are soilborne roundworms that attack the root system of tomatoes, preventing water and nutrient uptake. Tomatoes will often be stunted before they exhibit wilting by nematodes. Many tomato varieties are resistant to root-knot nematode, a nematode that causes swellings or galls on the roots. The letter "N" will follow the variety name to indicate nematode resistance. If you have any questions or suspect nematodes are causing wilt, please contact your local extension specialist or the MU Extension Nematology Lab at 573-882-0186.

Tomato Spotted Wilt Virus

Tomato spotted wilt virus: We are seeing a lot of tomato spotted wilt virus this year because the population of tobacco thrips, which transmit the virus, is high. This virus has distinct leaf and fruit lesions accompanied by wilting.

Walnut wilt: If tomatoes are planted close to walnut trees or related species including butternut or pecan, a chemical (juglone) released by these trees will incite wilting of tomatoes. This can occur in a relatively short time even though there is ample soil moisture. The wilting may occur on all or part of the plant.

Excessive water: Waterlogged soils can create problems with tomatoes. Roots need oxygen, and soils that do not drain well will inhibit root growth causing the plant to die. Therefore, tomatoes should be placed on raised beds to promote soil drainage.

Physiological leaf roll: Leaf rolling is a physiological disorder and is not a pathological disease. Some cultivars (e.g., ‘Mountain Spring’) are more likely to roll their leaves. Symptoms include an upward cupping of the lower leaves. Physiological leaf roll can be exacerbated by late or severe pruning of tomatoes.

Other potential causes: If you irrigate, check affected plants for being in a "rain shadow" of taller plants. If you are using drip irrigation, some emitters may not be working properly. Tunneling by moles can disturb roots and also divert irrigation water. Samples can be sent through your local Outreach and Extension Center to the Plant Diagnostic Clinic on campus for positive identification of the problem. Your observations of the symptom patterns and the plant parts affected will help in their diagnosis.

Lewis Jett, Assistant Professor & State Vegetable Crops Specialist, UMC (573) 884-3287