While open-field tomatoes are just getting started in parts of this region, high tunnel and greenhouse tomatoes have been in the soil for quite a while now to get that early start on the season. So we are seeing a number of diseases appearing on tomatoes produced in high tunnels and greenhouses. Some of the diseases are soilborne, some airborne, and some spread by insect pests. Here are some examples we’ve seen so far this spring:
Tospoviruses – Tomato spotted wilt virus (TSWV) and Tomato chlorotic spot virus (TCSV). Symptoms include yellow and/or necrotic spots on leaves, as well as purplish-to-brown spots or streaks on petioles and stems. Plants may wilt and new growth may be very stunted. Fruit symptoms appear as blotchy yellow or necrotic spots or ringspots, depending on the virus. Tospoviruses are transmitted by thrips, which acquire the virus while feeding as larvae on infected plants. Adult thrips do not acquire these viruses, although thrips that are infected retain the virus for their lifetimes and can pass the virus on to their progeny. We can test for TSWV quickly with a commercially available immunostrip test, but TCSV requires a more time-consuming lab test. Once the disease is observed in a greenhouse or high tunnel, the infected plants should be removed and destroyed, or they will serve as a reservoir of the virus. Thrips populations must also be controlled. More information can be found in last year’s post on TSWV in peppers and tomatoes and on OSU’s Vegetable Disease Facts website.
Botrytis gray mold. This is a disease caused by the fungus Botrytis. Botrytis spores are windborne and are most likely to cause problems when the weather is cool, skies are overcast, and humidity in the greenhouse or high tunnel is high. Leaves, stems, petioles and fruit can be infected. Fuzzy brown-to-grey moldy growth on infected tissue is the telltale sign of grey mold disease. The disease is managed by cultural practices such as reducing humidity in the structure by ventilation, pruning plants adequately, maintaining fertilization, especially calcium, at a level optimal for plant growth, and removing dead and dying tissue to reduce the spore load. Several fungicides are labeled for use on tomatoes in greenhouses or high tunnels, including Fontelis, Botran, and Scala. See the Midwest Vegetable Production Guide for more information. Products such as Oxidate, Fracture and various biocontrols may reduce disease pressure and be acceptable for organic growers as well. See our Botrytis gray mold factsheet for pictures of symptoms and an in-depth discussion of disease management.
Pith necrosis. Pith necrosis is also a disease favored by cool weather, so it is more of a problem in the spring and 
early summer in tomatoes produced under protected culture than in mid-summer. It usually appears to be randomly distributed in the tomato crop, and is associated with high nitrogen in soil. Pith necrosis is caused by several species of bacteria, most commonly Pseudomonas spp., all of which are common soil inhabitants. Affected plants may wilt and die; and brown to black lesions may be seen on the outside of the stem or petioles. The bacteria can be spread from plant to plant through contaminated tools or hands, so sanitation is very important in the management of the disease. Diseased plants should be removed from the greenhouse or high tunnel and destroyed. Pith necrosis can be diagnosed by splitting the stem lengthwise with a sharp knife and checking the pith in the center of the stem. Diseased pith is dark brown in color. You can find more information and pictures of symptoms on our Vegetable Disease Facts-Pith Necrosis website.
Verticillium wilt. This disease is caused by the fungus Verticillium, which lives in soil and infects plants through the roots. The disease is not usually a major problem in tomatoes in open fields as long as adequate crop rotation is practiced and resistant varieties are used. Adequate crop rotation (out of tomato/pepper/eggplant for at least 3 years) is not always practiced in greenhouses and high tunnels, so we are seeing Verticillium wilt and other soilborne diseases emerging in these tunnels. Further, many heirloom varieties popular for high tunnel/greenhouse production are very susceptible to Verticillium wilt. There are no effective fungicides available to control this disease. We are working to adapt a relatively new approach called anaerobic soil disinfestation (ASD) to protected culture systems, which has the potential to reduce disease pressure caused by Verticillium and other soilborne pathogens. (Verticillium photos by A. Testen).
White mold. This disease is caused by the fungus Sclerotinia, and is most common under cool, wet, high humidity conditions. See my post a year ago on white mold management in high tunnel tomatoes for details.