This year we've had more calls than usual about wilting and dying tomato plants. Wilt diseases of tomato can be caused by three different pathogens, as well as a toxin produced by walnut roots. Walnut wilt produces similar symptoms to the wilts caused by pathogens, including brown discoloration of internal woody tissue. For more on walnut wilt, try the University of Illinois Extension's Hort Solutions Web site (http://www.ag.uiuc.edu/~robsond/solutions/hort.html). Other noninfectious causes of wilting of tomatoes include flooding or rotting of roots (common this year), some herbicide injuries, and mechanical injury to the roots or stem.
The infectious vascular wilt pathogens of tomato include bacteria and two fungi. The two fungi are Fusarium and Verticillium. 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 might show symptoms, whereas leaves on the other side appear normal. Wilting could occur at or during the hottest part of the day or when the plants are stressed from dryness or a heavy fruit load. Infected leaves might dry up before the wilting is detected, so initial symptoms may appear as stems with dead leaves.
A key diagnostic sign for both fungal diseases is discoloration of the vascular (woody) tissues. The very center of the stem is the pith. The vascular tissue is the tissue between the pith and the outside of the stem. 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 are discolored. The discoloration is more of a grayish color with Verticillium wilt. However, this distinction is not always clear cut, so laboratory isolation is required to distinguish these two pathogens from each other and from walnut wilt. 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 sudden wilt of the plant without leaves first discoloring. The center of the stem (pith) will be water-soaked at first, then brown, and eventually may become hollow. If the plant wilts as described but does not have any discoloration of the stem, carefully dig up and inspect 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 someone asks, "Why do my VFN tomatoes have symptoms of Verticillium or Fusarium wilt?" The first possibility is that the symptoms are the result of something other than these two diseases, or it may be that the plants were not actually the variety that they were labeled. Another possible explanation is that a race of the fungus is present in your garden to which this variety does not have resistance.
These are very difficult diseases to control. The first step in managing the problem is to be certain you have accurately identified the cause of the wilt. Hopefully this article will help. Further details on wilts of tomato can be found in Report on Plant Diseases No. 929. Details on Verticillium wilt on many crops can be found in RPD No. 1010.