This summer many gardeners have been puzzled by the sudden wilting and death of tomato plants. Possible causes of wilting include lack of water, vascular wilts, tomato spotted wilt virus, walnut toxicity, or stalk borers.
- Lack of Water
Tomato plants require approximately 1 inch of water per week. Plants may wilt badly when soils are dry, but will revive rapidly when they are watered. A thorough watering once a week during hot, dry weather should be sufficient. Apply water directly to the soil around the base of the plants with a garden or soaker hose. If an overhead sprinkler is used to water the tomatoes, water the plants in the morning to reduce foliar disease problems. Fortunately, generous rains during the past two weeks have alleviated drought problems in most Iowa gardens.
- Vascular Wilts
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 wilts 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, Burpee VF, and Celebrity.At this time, the vascular wilts are thought to be responsible for most cases of wilting tomatoes in Iowa.
- Tomato Spotted Wilt Virus
Tomato spotted wilt virus (TSWV) can cause stunting, wilting, bronzing of foliage, and brown or green rings on fruit. A virus disease, TSWV can infect plants in the greenhouse or in the field. Infected plants cannot be cured and should be removed from the garden. No tomato varieties are resistant to tomato spotted wilt virus.
- Walnut Toxicity
Black walnut trees produce a toxic material (juglone) that can injure and kill solanaceous crops (tomatoes, potatoes, peppers and eggplant) and other juglone-sensitive vegetables in the garden. Symptoms of walnut toxicity include stunted growth, yellowing and wilting of foliage, and death of susceptible plants. Juglone is present in all parts of the black walnut tree (fruits, leaves, branches and roots). 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 leaves and other plant debris by soil microorganisms also releases juglone. Living roots exude juglone into the surrounding soil. Generally, the greatest concentration of juglone in the soil exists within the dripline of walnut trees. Nothing can be done to save juglone-damaged tomato plants. Simply remove and destroy dead plants. Gardeners who have large walnut trees near their gardens should consider alternate sites. If alternate sites are unavailable, plant tomatoes and other susceptible plants 20 to 25 feet beyond the dripline of walnut trees to minimize walnut toxicity problems. Corn, beans, onions, beets, and carrots are tolerant of juglone and can be planted closer to walnut trees provided the area receives sufficient sunlight. Walnut trees that are 75 to 100 feet from the garden shouldn't be a big threat to tomatoes and other juglone-sensitive vegetables.
- Stalk Borer
The stalk borer is an insect pest that attacks a wide variety of plants including tomatoes. 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 tomatoes, potatoes, and peppers in the vegetable garden. An individual stalk borer may damage more than 1 tomato plant. 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 vegetable gardens may also help control the pest. Stalk borers cannot be effectively controlled with insecticides.
This article originally appeared in the July 22, 1992 issue, pp. 1992 issue, pp. 127-129.