Hi Jim,

Thank you for your question. In gardening there never seems to be one simple answer and the question of the curling of tomato leaves is a perfect example.  It can be caused by several different factors.  Environmental stress is the most common cause. Tomato leaves can curl from high temperatures and last week was a very hot one here in central Ohio. As long as the leaf curl is confined to just the top few leaves and the rest of the plant appears vigorous, your tomato should recover from a temporary heat-induced leaf roll.

Another factor may be excessive nitrogen fertilization which can also cause tomato leaves to curl. Excessive nitrogen will have another side effect, as it can favor the growth of new leaves and shoots at the expense of setting fruit. Check your fertilizer for nitrogen content.  You should use a balanced fertilizer. 

Watering also needs to be considered.  Aim for about two inches of water per week for in-ground plants.

Also, the degree of curling depends on the tomato variety. Heirloom varieties may have more leaf curl than hybrid tomatoes. Indeterminate varieties often show more leaf curl than determinate varieties.

The attached article goes into greater detail about causes of leaf curl.  I hope you will find it helpful.

Tomato Leaf Curl // Missouri Environment and Garden News Article // Integrated Pest Management, University of Missouri