There are three causes of leaf roll:
- Physiological leaf roll which may be caused by stresses such as too much water, excess nitrogen, and transplant shock. Often the plants recover and the leaf roll has no affect on the plant's productivity.
- Herbicide damage may also be a direct result of leaf roll symptoms. Fruit may be deformed but plants can recover if they are not exposed to too much. It is often a drift of spray from a neighbours garden or nearby farmer spraying his field that can cause the problem.
- The last possibility is a viral infection such as yellow leaf curl virus caused by sap sucking insects such as aphids. Once a plant has the virus its growth also becomes stunted and there is no cure.
There is little that can be done to avoid the second and third reasons (above) for leaf roll. Also, stress is difficult to avoid if you grow outdoors but stress is managable whereas herbicide damage and viruses aren't.