Causes of Tomato Leaf Fold

Physiological Causes

• Frequently, environmental conditions are at the root of leaf folding and curling. A sudden heat wave can cause the leaves of young plants to fold up because the root system isn't large enough to support lots of leafy growth during hot weather. A bout of cold, rainy weather can also shock the plant into folding its leaves. Drought conditions, excessive nitrogen fertilizer or even overpruning can cause leaves to fold. To prevent damage, plant healthy seedlings outdoors only after the weather has predictably warmed in the spring. Harden off the seedlings by placing them in a sheltered outdoor location for a few days before planting. Avoid overpruning young plants and go easy on the nitrogen fertilizer.

Diseases

• Two viruses are often associated with leaf folding. Tomato mosaic virus causes the leaves to cup or fold. You'll also notice a mosaic pattern on the leaves and the tomato fruit turns brown on the inside. Tomato yellow leaf curl virus causes the leaves to fold or curl. They may turn green or develop purple veins. Growth is stunted and the plants produce little fruit. Tomato yellow leaf curl virus is spread by whiteflies. There is no cure for these diseases. Instead, remove and destroy the plants. In the future, install floating row covers over young plants to protect them from insects. Plant disease-resistant varieties. Weeds sometimes harbor the viruses, so keeping the garden weed-free can reduce problems.

Herbicide Injury

• Tomato plants are very sensitive to herbicide exposure. Herbicides sometimes drift onto the plants on breezy days. Even touching tomatoes with contaminated hands can cause injury. Plants can also be injured by contaminated mulches of grass clippings, straw or manure. Broad-leaf weed killers, such as 2, 4-D, cause the leaves to curl or fold. The plants become stunted and might produce little, if any fruit. To prevent damage, spray herbicides on calm, windless days. Wash your hands thoroughly after making an herbicide application, and avoid using treated grass clippings as a mulch. Plants that are damaged may recover eventually with continued care.

General Recommendations

• Regardless of the cause of the leaf folding, healthy tomato plants are better able to survive and outgrow problems. Always select stocky, young plants with dark green foliage. Avoid tall spindly ones with pale leaves. Tomatoes need at least six to eight hours of sunlight daily. They also need rich, well-draining soil. Tomatoes are subtropical plants that thrive in warm weather. Floating row covers and plastic cloches can help moderate temperature to keep them healthy.