Curled or rolled leaves on a tomato plant can be a sign of physiological problems, herbicide damage or viral infections, according to the University of Clemson Cooperative Extension. The possible physiological reasons include transplant shock, change in temperature at night, drought, severe pruning and root damage.
The curled leaves could also be a sign of excessive heat, moisture or nitrogen. These physiological problems are normally first seen on the lower leaves, which gradually begin cupping and then rolling in towards the main vein. The leaves usually become thicker and take on a leathery texture as well. If left untreated, these problems can spread to all of the plant's leaves.
Herbicide damage is another common cause of leaf curling, which leads to the leaves curling downwards. Depending on the amount of exposure, the plant may become twisted or distorted, the fruit may grow deformed or the stems may turn white and split open. If the plant was exposed to a large amount of herbicide, it is likely to eventually die.
There are also many viral infections that can cause tomato leaves to curl. One of the most common viral infections is known as tomato yellow leaf curl virus, which is transmitted to the plants by whiteflies. This virus causes the leaves to turn a pale green color and begin curling. There is no cure for this virus, so any affected plants need to be removed and destroyed.