Don't be alarmed if you discover curled-up leaves on your otherwise healthy-looking tomato plants. Leaf curl, also called leaf roll, usually does not affect the quantity or quality of the tomatoes, according to the University of Illinois Extension service. Causes of leaf curl can include a physiological condition, exposure to herbicides or a virus affecting the tomato plant. Gardeners can take steps to prevent leaf curl, but the measures depend the source of the problem.
Physiological Leaf Curl
Rainy weather combined with cool temperatures can cause tomato plant leaves to curl or roll up on themselves. The condition, known as physiological leaf curl, can also affect seedlings soon after you transplant them outdoors. The problem does not require treatment because it does not inhibit plant growth or harm the fruit, says Judy Sedbrook, Colorado master gardener with the Colorado State University Cooperative Extension. Another type of physiological leaf roll--termed "non-parasitic leaf roll"--occurs in response to over-pruning or lack of water. The leaf curl will usually disappear in a few days, once the plant recovers from pruning or its water supply stabilizes.
Herbicide-Caused Leaf Curl
Tomato plants exposed to herbicides can show signs of leaf curl, and in severe cases, the leaf surface becomes white and the leaf itself turns thick and brittle. The most common herbicidal cause of leaf curl comes from 2,4-D, which is often used to treat lawns or crops for weeds. To minimize tomato plants' exposure to herbicides, never spray your plants with same sprayer that you use on your lawn. Don't use treated grass clippings as mulch around tomato plants. Herbicidal leaf curl can result in a harder-than-normal or misshapen tomato fruit, but unless the exposure to the herbicide is prolonged and severe, most plants will recover, especially if you provide them with frequent watering.
Leaf Curl Virus
Sucking insects like the sweet potato whitefly and aphid transmit a disease known as the tomato yellow leaf curl virus. The condition usually affects tomatoes that grow indoors. Leaves that curl upward occur in the first stages of infection, then the leaves curl downward and become yellow. Since no cures exist for the virus, the best way to prevent yellow leaf curl virus is to control insect infestations and select healthy, insect-free transplants for fruit production.