Curling or rolling of tomato leaves can be caused by various factors, including environmental stresses, viral infection, and herbicide damage. To determine which factor is the culprit, it pays to take a close look at the plant(s). Which leaves are rolling – old leaves, new leaves, all leaves? What direction do the leaves roll – upward or downward? Are any other parts of the plant, including fruit, exhibiting symptoms?
Physiological Leaf Roll
Excessive moisture and nitrogen, heat, drought, severe pruning, root damage and transplant shock are some of the environmental factors that can cause physiological leaf roll in tomatoes. Initial symptoms are usually apparent in the lower leaves with an upward cupping of leaflets followed by an inward lengthwise rolling of the leaflets toward the mid-vein. The affected leaves tend to become thickened and have a leathery texture, but retain a normal, healthy green color. Over time all of the leaves on the plant may be affected.
Interestingly, vine tomato (indeterminate) varieties tend to exhibit physiological leaf roll more often than bush tomato (determinate) varieties. While this condition can occur at any time of the growing season, it usually occurs as spring weather shifts to summer. The good news is that the condition has minimal impact on tomato fruit production and plant growth. By properly hardening off tomato seedlings before planting in the garden, maintaining a consistent moisture level in the soil, and avoiding over fertilization, excessive pruning and root damage during cultivation, one can go a long way toward preventing tomato plants from developing this physiological problem.
Some viral infections also cause leaf rolling in tomatoes. When tomato plants are infected with Tomato yellow leaf curl virus (transmitted by whiteflies), new leaves become cupped and pale green in color. In addition the entire plant may exhibit stunted growth, yellowing leaf edges, purplish veins on the undersides of leaves, and decline of fruit production. A second virus, Tomato mosaic virus, causes rolling of leaves, but other symptoms, including mottled-coloring of leaves, small leaflets, and internal browning of infected fruit, distinguish it from physiological or herbicide-induced leaf roll.
There is no treatment for virus-infected plants. Removal and destruction of plants is recommended. Since weeds often act as hosts to the viruses, controlling weeds around the garden can reduce virus transmission by insects. As some viruses are transmitted mechanically on garden tools, it also helps to disinfect tools that have come into contact with diseased plants.
When tomato plants are exposed to the herbicide, 2,4-D, typical symptoms include downward rolling of leaves and twisted growth. In addition, stems may turn white and split; fruit may be deformed. Depending on the level of exposure, the plant may or may not survive.
Herbicide injury cannot be reversed, but if the plant is not killed, new growth may be normal. Always be very careful when spraying an herbicide, as it may drift much further than anticipated.