It is common at this time of year to see many odd symptoms on tomato plants. Wilt diseases may be a problem in your garden or production area. Wilt diseases of tomato can be caused by three different pathogens as well as by a toxin produced by walnut roots. Walnut wilt produces a brown discoloration of internal woody tissue similar to the infectious wilts. We do not see walnut wilt often, but it does occur. It is a reaction of the tomato to the toxin in the roots, so plants must be within the root zone of the walnut. Check out the Extension solutions page at http://www.ag.uiuc.edu/~robsond/solutions/hort.html for more information on walnut wilt. Other noninfectious causes of wilting of tomatoes include flooding or rotting of roots, some herbicide injuries, and mechanical injury to the roots or stem.
The infectious vascular wilt pathogens of tomato include bacteria and two fungi. Fusarium and Verticillium are the fungi that cause the very similar Fusarium wilt and Verticillium wilt diseases. Infected plants might be somewhat stunted; and leaves turn yellow and die, often starting at the base of the plant and progressing upward. Leaves on one side of the plant may show symptoms, while leaves on the other side appear normal. Wilting may occur at or during the hottest part of the day or when plants are stressed because they are dry or bearing a heavy fruit load. Infected leaves may dry up before wilting is detected, so the first symptom may be stems with dead leaves. The most diagnostic symptom of these diseases is discoloration of the vascular (woody) tissues. With Fusarium wilt, the vascular tissue of stems and petioles throughout the plant becomes brown to reddish brown; with Verticillium wilt, only the lower stem tissues take on a grayish color. This distinction is not always clear cut, however, and laboratory isolation is required to distinguish these two pathogens positively. Both fungi are soilborne and infect plants through root systems, and both are able to survive in soils in the absence of a susceptible tomato plant for many years. Fusarium can also be seedborne.
Bacterial wilt is caused by a Pseudomonas species and affects potatoes, eggplants, and peppers, as well as tomatoes. This pathogen causes a plant to wilt suddenly without its leaves discoloring first. The center of the stem is water soaked at first, then brown, and eventually it may become hollow. If the plant wilts as described but does not have any discoloration of the stem, carefully dig it up and inspect its roots for root rot. Roots injured by excess water and/or root rot will also cause these symptoms.
Disease resistance is the most common and economic means of controlling the fungal diseases. Tomato varieties labeled “VFN” have resistance to Verticillium, one or more races of Fusarium, and nematodes. Occasionally, I hear: Why do my VFN tomatoes have symptoms of Verticillium or Fusarium wilt? The first possibility is that the symptoms are caused by something other than these two diseases. It may be that the plants are not actually the variety that they were labeled. Another explanation is that a race of the fungus is present in your garden to which this variety does not have resistance. The Plant Clinic does not have the ability to identify fungal races.
Rotation out of an affected area for five to seven years can help reduce the incidence of Verticillium wilt, but shorter rotations will be of little benefit. Removing infected plant parts, including roots, might reduce the buildup of inoculum. Research shows that using nitrate forms of nitrogen can help reduce the severity of Fusarium wilt, while ammonium forms promote disease development. Because low soil pH also favors Fusarium, increasing soil pH to a range of 6 to 7 can help control Fusarium wilt. Higher than pH 7.5 increases problems with Verticillium wilt. Details on the wilts of tomato can be found in Report on Plant Disease No. 929. Details on Verticillium wilt on many crops can be found in Report on Plant Disease No. 1010.