What Does It Mean When the Leaves Curl on a Plant?

Fungus and Bacteria

• Insects, including leafhoppers and spittle bugs, transmit bacterial leaf scorch, which affects some trees. For example, sycamore trees (Platanis occidentalis), which grow in U.S. Department of Agriculture plant hardiness zones 4b through 9a, are susceptible to this disease, which interferes with water transport and causes leaves to turn brown and curl. Prune infected branches and remove heavily infected trees.

  In rainy spring weather, peach leaf curl disease causes stunted twig growth and curled or puckered leaves in trees related to peach trees (Prunus persica). Thoroughly spray a peach tree once during dormancy with a dilution of 3 teaspoons of copper fungicide per gallon of water. Peach trees grow in USDA plant hardiness zones 5b through 8.

Viruses

• Tomato yellow leaf curl virus is a serious disease in tomato plants (Lycopersicon lycopersicum) and other vegetables, states the University of California Integrated Pest Management Program. Tomato plants grow as perennials in tropical regions and as annuals in USDA plant hardiness zones 3 through 9. The Bemisia whitefly spreads the virus, which causes small, curled leaves with yellow margins. Raspberry leaf curl virus affects raspberry plants (Rubus ideaus and Rubus occidentalis), which grow in USDA plant hardiness zones 4 through 9. The raspberry aphid spreads this virus, which causes small, dark, leaves that curl downward.

  Spray insecticidal soap on whitefly- or aphid-infested plants, and remove plant debris after the growing season New plants should not grow near soil where infected plants grew.

Insects

• Some tiny insects such as aphids, psyllids and scales suck the sap from plants and cause new leaves to curl. Heavy infestations of these insects can cause other severe symptoms. For example, a heavy scale infestation can cause branch dieback and brown, scorched leaves. You can remove light infestations of these sucking insects from leaves and stems with a strong stream of water or by hand. Insecticidal soap spray smothers and kills the insects in heavily infested plants.

Other Causes

• Environmental conditions, poor cultural practices and herbicide fumes can cause curled leaves in plants. For example, dry wind and insufficient water cause brown, curled leaves. To prevent leaf curl from these conditions, you can shelter the plants from wind or work in 3 or 4 inches of compost to improve water retention in the soil. Tomato plants are sensitive to fumes or drift from the herbicide 2, 4-D, which causes leaf cupping and curling. Provide sufficient water for your specific plants to help them recover from herbicide drift.