Tomatoes are subject to many conditions that can cause wilting. Some of these are serious, like fusarium and verticillium wilts, and some are not, such as a simple lack of water or extreme heat. Knowing how to diagnose the wilting is an important tool for preventing or curing the problem.
Fusarium and verticillium wilts are disorders caused by fungal agents present in the soil. Newly affected plants may wilt during the heat of the day and seem to recover in the evening or overnight. However, the wilting will gradually become worse, and the plants may die. Unfortunately, there is little that can be done to prevent this problem, other than to plant varieties that are resistant to these pathogens. Tomatoes that have this resistance will be labelled as "V" for verticillium and "F" for fusarium. Some types will have both codes, and some will have others as well. The codes will be present on the seed package or on the identifying tag in case of seedlings. Tomato spotted wilt virus (TSWV) can also cause wilting as well as brown spots on the leaves and stems. There is no cure for this disease, and no varieties with increased resistance. Affected plants should be removed.
Walnut trees secrete a substance called juglone that can stress or even kill some types of plants, including those in the solanaceous family like tomatoes, peppers and eggplants. Tomatoes planted near walnut trees may exhibit wilting during the day when temperatures are at their hottest. The only way to avoid this is not to plant anything near walnut trees. Planting at least 25 feet from the outer reach of the walnut tree's spread should prevent this problem.
Tomatoes with damage to their stems due to insect pests like cutworms, stem borers or aphids may exhibit wilting. Check wilting tomatoes for infestations of these and other insects and treat with an appropriate insecticide. Also, plants damaged during transplant may exhibit wilting. Newly transplanted tomatoes may exhibit wilting during the hottest parts of the day until well established.
Overfertilizing your plants can cause them to be unable to utilise water in the soil. This can also cause wilting. Carefully follow directions on any fertilisers used; do not exceed dosages.
Lack of Water
Tomatoes require lots of water. A mature tomato plant may need as much as a gallon a day. Rainfall of approximately one inch per week should be sufficient. Tomatoes like an even level of moisture, preferring consistently moist soil conditions. Check the soil to make sure they are getting enough moisture. An underwatered tomato will recover from a condition of wilting very quickly once water is applied.