A problem occasionally encountered by home gardeners is the sudden wilting of tomato plants. The three most likely causes of wilting are vascular wilts, stalk borers, and nearby walnut trees.
The initial symptoms of Verticillium and Fusarium wilts are wilting of the plant leaves during the heat of the day. Affected plants often recover in the evening or overnight. Gradually, however, the wilting becomes progressively worse and many plants eventually die.
Verticillium and Fusarium wilt are caused by soil-borne fungi that invade tomato plants through injured roots. The fungi spread into the water-conducting tissue (xylem) in the stem and block the flow of water to the foliage. Foliage of affected plants turns yellow, then wilts and dies. A cut through the lower stem of a dead plant often reveals a brownish discoloration of the vascular tissue.
There is nothing that can be done for plants that have Verticillium or Fusarium wilts. Plants that die should be removed and destroyed. Crop rotation is of limited value as the vascular wilt fungi may survive in the soil for several years. The use of resistant varieties is the most practical way for home gardeners to prevent losses due to wilts. Resistant varieties may become infected but many plants survive and produce an acceptable crop. Resistant varieties are available in seed catalogs and at garden centers. The letters V and F following the variety name in seed catalogs or on seed packets denote varieties that are resistant to Verticillium and Fusarium wilts. Wilt resistant tomato varieties that perform well in Iowa include 'Jetstar,' 'Better Boy,' and 'Celebrity.'
The stalk borer is an insect pest that attacks a wide variety of plants including tomatoes, potatoes, peppers, corn, hollyhocks, and dahlias. The larva (caterpillar) bores into the stem and tunnels inside the stalk. (The entrance hole is small and often difficult to locate.) Affected plants wilt and often die. However, stalk borer damaged plants that are given good care may survive.
The stalk borer is a purple and cream striped caterpillar with a solid purple band around its body 1/3 of the way back from its head. It is an early season pest that moves from tall grassy weeds and occasionally attacks plants in the garden. An individual stalk borer may go from one plant to another, damaging several plants. The adult is an inconspicuous grayish brown moth.
Tomato plants that die should be pulled and destroyed. The destruction of the plants may also kill the stalk borer. Cutting or mowing tall weedy areas around gardens may also help control the pest. Stalk borers cannot be effectively controlled with insecticides.
Black Walnut Toxicity
Since ancient times, scholars have suspected that walnuts have harmful effects on nearby plants. In the 1880's, scientists isolated a compound called juglone from the fruit of walnuts. They demonstrated that injury and sometimes death result when this phytotoxic material interacts with susceptible plants.
The sources of juglone in the soil include both living and decaying plant material. Rain droplets leach juglone from the buds, leaves, and twigs. The decomposition of plant debris by soil microorganisms also releases juglone. Living roots exude juglone into the surrounding soil. Vegetables susceptible to juglone include tomatoes, peppers, and eggplants.
Symptoms of black walnut toxicity include reduced growth, yellowing of foliage, wilting, and eventual death of the plant. To prevent black walnut problems, plant tomatoes well beyond the dripline of walnut trees. Do not use wood chips derived from walnuts as a mulch around tomatoes or other sensitive plants.
This article originally appeared in the June 25, 1999 issue, p. 86.
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