Tomatoes are easy-to-grow plants commonly grown in backyard vegetable gardens. However, the plants are susceptible to a number of problems, including curled leaves. If the foliage of your tomato plants begins to curl, diagnose the cause of the problem so you treat it accordingly. Leaf curl is caused by a number of reasons, including improper cultural control, pest damage, chemical damage and Tomato Leaf Curl Virus (ToLCV). Treat the problem immediately so your plant regains health and vigour and continues to produce flavourful tomatoes.
Inspect your tomato plants for sucking insects such as leafhoppers that cause crinkled or curled foliage or potato aphids that curl leaves and cause foliage to drop prematurely. Leafhoppers are green, wedge-shaped pests that measure 3/8 inch long, while pink or green aphids measure 0.12 to 0.16 inch long. Spray infestations with antibacterial soap or neem oil to dislodge the pests. Remove severely infested or damaged plants to prevent the pests from spreading.
Water the tomato plants regularly, especially during warm periods and severe drought to keep them from dehydrating and curling their leaves. The leaves of water-stressed plants curl inward to conserve moisture. Similar to underwatering, the foliage of overwatered tomato plants also curls due to excess soil moisture or poor soil drainage. Plant tomatoes in well-draining soil to prevent waterlogged conditions and provide each plant 1 to 2 inches of water every week.
Examine the curled foliage of the plants for yellow margins that indicate the Tomato Leaf Curl Virus. The virus is transmitted by whiteflies that hop from one plant to another, spreading the infection. Spread 60-mesh or finer netting over young transplants to prevent access by whiteflies. Apply an insecticide containing imidacloprid or carbamate to control infection. Collect plant debris from around the tomato plant and destroy it.
Inspect leaves of the tomato plant soon after an application of 2,4-D herbicide to nearby weeds to determine whether the tomato leaves curled. The herbicide causes severe damage to tomato plants, causing the leaves to twist, cup and curl upward. Water the infected plant frequently so it outgrows the injury. Cover tomato plants with a tarp when spraying undesirable plants nearby to prevent drifts of the herbicide.
The foliage sometimes curls or rolls up on itself in rainy weather and/or cool temperatures. Called physiological leaf curl, the condition also affects seedlings soon after you transplant them outdoors and does not require treatment since it does not damage the plant or affect fruiting. Avoid planting tomato varieties such as "Big Boy" and "Beef Steak" that are susceptible to leaf curl if you have clay soil.