When a tomato plant wilts the first thought that comes to mind is that it is water stressed, and quite frequently that is the problem. A lack of water, particularly in hot summer weather is the most common cause of tomato plants wilting. So long as you haven't neglected the plants for any extended period of time, water stress is is easily rectified.
Watering is critical to the development of Tomatoes. Water slowly and deeply . During hot dry spells water deeply, allow the soil to dry and water again. Tomatoes require at least an inch of water weekly, slightly more in sandy soils. In very hot weather they need up to 2 inches weekly.
At times, even when the plants have received adequate moisture they will still wilt. When you are certain the plants are well irrigated and they still continue to wilt it is quite likely that your tomato plants have a disease or nutrient deficiency. Wilts of tomato plants not caused by water stress are generally fungal diseases. Verticillium and Fusarium wilts are the most common. The symptoms of both diseases are very similar and rendering a correct diagnosis is not always easy.
Only a laboratory test can determine beyond the shadow of a doubt which wilt you have, a lab test of course for home growers is simply not feasible. Verticillium wilt starts with a general or blotchy yellowing of the lower leaves and a wilting of younger shoots when exposed to direct sunlight. Initially, the wilted shoots will recover over night.
Fusarium Wilt causes drooping and yellowing of the lower leaves on one stem. These symptoms sometimes appear on only one side of the plant. Yellowing and wilting of the leaves occurs, and the plants may eventually die. When the stem is cut , plant tissues under the surface are frequently discolored brown. Favored by soil temperatures around 80 F, and a low soil pH, Fusarium wilt is more common in southern latitudes.
There is no actual proven cure for Fusarium Wilt. It remains in the soil for up to a decade. It is difficult to completely eradicate it. But there are measures that help to control it.
1. Use only sterile Potting Soil when starting your own seedlings.
2. Raising the Soil pH helps to keep it in check.
3. Use Resistant Varieties
Veryicillium starts with a blotchy yellowing of the lower leaves and a wilting of younger shoots when exposed to sunlight. At first the wilted shoots will recover over night. The leaf veins turn brown, followed by the shoots. The lower leaves drop. Leaves higher up turn a dull green in color and the edges curl upwards.
Once you have a Fusarium or Veticillium wilt on the Tomatoes there is no magic wand to get rid of it. Fungicides intended for the control of leaf blights have no effect on wilts which are internal infections.
Tomato Spotted Wilt Virus
TSWV is spread by Thrips . Spotted Wilt or TSW will greatly reduce the quantity and quality of the harvest. Reflective Mulch will reduce, but not eliminate losses from Tomato spotted Wilt . Management of Thrips will also reduce, but not eliminate TSWV.
Souther Blight is more easily diagnosed, allthough it has been confused with powdery mildew. The appearance of white mold at the base of the plant, followed by rapid wilting are the most common signs. Once again, once the plant has this disease it is not treatable - get rid of it and burn it or bag it don't leave it lying around and certainly do not compost it.
Tomato Bacterial Wilt
Tomato Bacterial Wilt is not as common as other diseases but can also cause a tomato plant to wilt and die off. They wilt and die fairly rapidly. The stem inside will be hollow, dark and watery. Reflective Mulch will reduce, but not eliminate losses from Tomato spotted Wilt . Management of Thrips will also reduce, but not eliminate TSWV
Tomato Yellow Leaf Curl virus
This virus is easier to diagnose, it is transmitted by sap sucking insects. Infected plants are stunted and erect. It can lead to a severe stunting of the plants growth with little or no yield at all. Leaves are smaller than average and curl upward with marginal yellowing.
If you've lost a number of plants to any of the fungal diseases you may want to 'solarize' the soil. Soil solarization is a method for controlling soilborne disease and pests using the suns radiant energy and the high temperatures produced.
A clear plastic tarp is placed over the effected soil surface for a month or two, experts recommend 4-6 weeks, but longer can't hurt. The clear plastic tarp traps the sun's energy in the soil and heats the top foot to 1.5 feet of the soil. Temperatures under the tarp and in the soil surface sometimes exceed 130 -1 40 F which effectively kills off pathogens, nematodes, insects and their larvae in the soil as well as nematodes. Plants should be kept well watered durring this time.
Insects in addition to spreading disease to your tomato plants are on occasion the cause for plants wilting. Aphids , white flies, stalk borers, root knot nematodes can also deplete the plants resources and cause wilting.
On rare occasion other plants which are alleopathic may be the culprit. Allelopathy is a natural phenomenon where one plant produces biochemicals that exert an influence over other plants. These biochemicals are known as allelochemicals and can have both beneficial or detrimental effects on neighboring plants, animals, insects and organisms. See - Alleopathic Plants