Tomato leaves may roll for three main reasons: physiological stress, viruses, or herbicide damage. Of the three, physiological stress is the easiest to remedy.
Physiological stress refers to anything in the environment that is not optimal for tomatoes: too much or too little water, high winds, fertilizer burn, root damage, and transplant shock. The rolling first appears on the lower leaves and involves cupping inward. However, the leaves retain their green color. They may become leathery and thickened. Over time, all the leaves of the plant may be effected.
The most common time for leaf curl from physiological stress is as spring turns into summer. Vine tomatoes seem more vulnerable to leaf curl than do bush tomatoes.
Fortunately, this condition has minimal impact on tomato production. The tomato plant can recover if you maintain a consistent moisture level, are careful not to over fertilize (expecially with nitrogen), protect the root zone of the tomato and properly harden off seedlings. It will probably continue to produce fruit even if the leaves do not uncurl, and new growth will be normal if the stressful situations are eliminated.
The second most common reason for tomato leaf roll is a virus. There are two main tomato viruses that cause leaf roll.
Tomato yellow leaf curl virus causes new leaves to become cupped and pale green in color. The entire plant may be stunted, show yellowing leaf edge, purplish veins on the undersides of leaves, and a decline in fruit production.
A second kind of virus, tomato mosaic virus, also causes rolling of the leaves. However, it causes mottled-coloring of leaves, small leaflets, and internal browning of infected fruit.
There is no cure for either of these viruses. If your tomato plant has them, the best thing to do is to pull the plant and destroy it. Do not compost it as this might spread the virus. The goal here is to contain the virus to as few plants as possible before it spreads. These viruses are spread by insects feeding on an infected plant, then moving to a healthy plant to feed and infecting it.
Weeds often act as reservoirs of disease for tomato plants so the elimination of weeds around the garden reduces the incidence of tomato viruses. There are no sick plants for the insects to feed on and spread the virus to your healthy tomato plants. Disinfecting tools used on weeds and diseased tomatoes before using them on healthy tomato plants can also prevent the spread of the virus.
The third most common reason for leaf curl is herbicide damage. When a tomato plant is exposed to the herbicide 2,4,D, leaves curl downward (as opposed to upward for physiological stress). In addition, the vine may turn white and split and the fruit may be deformed. The plant may not survive the herbicide exposure. However, if it survives, new growth should be normal. Always be careful when using herbicides to prevent drift and other accidental exposure to desirable plants.