Curling or rolling of tomato leaves may 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, or all leaves? What direction do the leaves roll – upward or downward? Are any other parts of the plant, including fruit, that are exhibiting symptoms?
Physiological Leaf Roll
Excessive moisture and nitrogen, insufficient phosphorus, heat, drought, severe pruning, root damage from tilling or hoeing, early planting and transplant shock are some of the cultural and 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. To lessen the chance of physiological tomato leaf roll, apply the following cultural techniques:
- Choose determinate growing tomato cultivars.
- Properly harden off tomato seedlings before planting them in the garden.
- Transplant tomatoes once the weather has warmed sufficiently.
- Maintain a consistent moisture level in the soil by mulching around the plants.
- Avoid over-fertilization with nitrogen.
- Apply sufficient phosphorus as based on a recent soil test report.
- Avoid excessive plant pruning or root damage during cultivation by tilling or hoeing.
- Site tomato plants in the garden to receive late afternoon shade, which is the hottest time of day.
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 any tools, such as pruners, that were used on diseased plants.
When tomato plants are exposed to the 2,4-D herbicides, typical symptoms include downward rolling of leaves and twisted growth. In addition, stems may turn white and split, and the fruit may become 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.