What causes leaf curl on tomato plants?

Environmental stress from bouts of cool rainy weather, herbicide damage, severe pruning, sucking insects and viruses tend to cause tomato leaves to curl.

Yo-yo watering and severe pruning may cause a tomato plant temporarily to develop thick, leathery leaves that curl upwards. In this case, the plant shows no effects on flowering, fruiting or growth. The plant should re-adjust within the week.

Herbicide damage, particularly from 2,4-D, may cause tomato leaves to curl first downwards and then upwards. In this case, the plant may show such effects as cat-faced fruit, stunted growth and twisted leaves. If the exposure is mild or minimal, such effects aren't seen, and any damage will be outgrown.

Herbicide damage may take place a couple of days after the original lawn treatment. It also may occur if the treated lawn is mowed and the clippings are used as mulch.

Sucking insects such as the sweet potato whitefly [Bemisia tabaci] may remove the nutrient-filled products of photosynthesis from a tomato plant. That's why the leaves that they so attack end up yellow in color and deformed in appearance.

Insecticidal soap is an organic control to the pest while an insecticide with the active ingredient esfenvalerate is a more potent control that yields more immediate effects. Insecticidal soap may be used up to the day before harvesting the fruit. With esfenvalerate there must be a day wait between the treatment and the harvest.

Viruses may cause a tomato leaf to cup first downwards and then up. The most recent and most problematic is the Tomato Yellow Leaf Curl virus. It's transmitted by such sucking insects as the sweet potato whitefly, which should be treated as listed above in the paragraph. Accompanying damage to the rest of the plant depends upon when during the tomato plant's growth the virus is transmitted. So it's possible that flowers may or may not wither and that fruit may or may not set.

The presence of the virus is encouraged by such weeds as jimsonweed [Datura stramonium] and nightshade [Atropa belladonna]. As long as such weeds and such plants as the tomato share space, gardeners need to consider the possible continued presence of the Tomato Yellow Leaf Curl virus.

Leaf roll, or leaf curl, is a physiologic distortion that may develop with periods of cool, rainy weather. It cause the lower leaves to roll upward and become thick and leathery. Leaf roll does not affect plant growth or fruit production and requires no treatment.

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Tomato leaf curl can be caused by a variety of things of which bugs (usually aphids) are only one. Look at the underside of the leaves that are curling. If you don't see aphids, it will either be normal for the leaves to curl or you have a fungus/viral problem. Tomato leaves curl naturally if it's too dry, too hot, too humid or too windy. In other words, the leaves will tend to curl if growing conditions are not optimum for the plant. If it's too hot and dry, water more deeply. If it's too wet, water less frequently (make sure you water deeply though). You can tell if it's a viral or fungal problem by looking at the entire plant, especially the leaves. Tomato leaves differ in color dependent upon the plant. There are some varieties with deep green leaves and others have lighter green leaves. If the leaves are lighter in color than they should be for that variety of tomato, you probably have a viral or fungal infection. The same holds true if you have spotting on the leaves. Cut off a small branch and take it to your local nursery to see what they recommend for your area of the country.