Common Tomato Diseases
Last month I mentioned that an important criterion when choosing tomato varieties is disease resistance. Plant breeders have developed varieties that are not affected by diseases such as Verticillium and Fusarium wilt. However, there are several diseases caused by pathogens that no varieties are resistant to, and other diseases that are caused by environmental diseases rather than fungal, bacterial, or viral pathogens.
Bacterial wilt, also called southern bacterial blight, is one such disease. Bacteria in the soil enter the tomato plant through wounds caused by transplanting, insect feeding, or lateral root emergence. The bacteria proliferate and clog the xylem (water conducting vessels), which prevents the plant from taking up water from the soil and distributing water to where it is needed. Symptoms manifest as severe wilting despite sufficient soil moisture, often with leaves remaining green. Cutting a stem crosswise and dipping it in water will cause conspicuous oozing as the bacteria flow out of the ruptured xylem. There is no cure for this disease, so it is important to remove infected tomato plants. The only way to break the disease cycle is to rotate (i.e., switch) what crop you are growing in the bacterial-wilt infested bed. Crop rotation is effective because many pathogens have ‘preferences’ for a specific set of plants, and not others. If you plant something besides host plants, the disease organism cannot complete its life cycle and eventually goes away. Make sure to rotate crops from different plant families: it does not good to plant different but closely related species. Tomatoes are related to eggplants, potatoes, and peppers (family Solanaceae), so beds infested with bacterial wilt should be replanted with corn (grass family, Poaceae), beans (legume family, Fabaceae) or cabbage (mustard family, Brassicaceae).
Among the most common physiological disorders of tomato is blossom end rot. Blossom end rot is characterized by a leathery lesion on the bottom of developing tomato fruit that progresses into general tissue collapse. This dead tissue then rots from secondary infection from molds and other fungi. Tomato growers often assume that blossom end rot is caused by calcium (Ca) deficiency. This is nominally true, but adding egg shells to the soil will not necessarily correct the problem. Usually soil calcium levels are more than adequate, and it takes months for egg shells to break down completely for absorption by plant roots. Instead, the tomato plant cannot transport adequate calcium to the developing fruit because of uneven watering, especially during drought. Make sure your tomatoes are getting about one inch of water per week, and use mulch to help stabilize soil moisture levels. Soil acidity also affects how easily plants can take up calcium from the soil. Tomatoes prefer a pH around 6.5, so if you soil test (conducted for free by the N.C. Department of Agriculture & Consumer Services and kits are available at your local N.C. Cooperative Extension Center) indicates your soils are too acidic, add the recommended amount of lime to raise pH to appropriate levels. Excessive application of magnesium (Mg) or potassium (K) can also inhibit calcium uptake. If your watering is uniform, soil pH is correct, and you haven’t been overzealous with Epsom salts, then a side-dress application of gypsum (calcium sulfate) of 1-2 pounds per 100 ft2 can help reduce blossom end rot.
Tomato spotted wilt virus is another common disease that is difficult to control. The virus is spread by the feeding of insects called thrips, which themselves acquired the virus by feeding on other infected plants, often weeds. The virus causes small dark flecks on leaves, giving them a slight bronze color and a tendency to cup upward. On tomato fruit, concentric rings with a raised center give young green fruit a bumpy appearance. More mature fruit may exhibit concentric yellow rings. Because viruses are incurable, it is best to remove infected plants, and keep weeds near your vegetable beds under control to prevent them from becoming a source of inoculum. Using insecticides to control thrips is often ineffective, as the infection process happens so quickly.
If you have questions about these and other tomato diseases, or any other problem with your vegetable or flower beds, please contact your local N.C. Cooperative Extension Center.
Matt Jones is Horticulture Agent for North Carolina Cooperative Extension in Chatham County.