Leaves on tomato plants curl upward because of several reasons, including viral infections, an effort to resist the excessive moisture when the conditions are excessively cool and moist and an effort to conserve water during a dry spell. Learning to identify the signs and knowing the reasons behind the curling up of tomato leaves is the most important aspect for a successful treatment and prevention of this problem.
The virus in a viral infection can be transmitted through infected transplants or through whiteflies. After the infection takes place, it may take about 3 weeks without showing any symptom of the infection, but the most common sign of this infection is the curling up and yellowing of the tomato leaves. This infection retards the growth of the plant, leading to a failure of the development of the flowers.
During excessively cold conditions, tomato leaves normally curl upward and become leathery so as to resist the excessive moisture. The condition is very common during the time of setting fruits, and it is often visible on staked and pruned tomato plants.
On the other side, the dry spell, uneven watering and the high temperatures cause tomato leaves to curl upward so that they can conserve water. However, under these conditions, the leaves do not appear to be leathery.