Home & Garden Information Center

Crape Myrtle Diseases & Insect Pests

HGIC 2002

Printer Friendly Version

Crape myrtles (Lagerstroemia indica) are essentially trouble-free small trees. The most common problems include powdery mildew, Cercospora leaf spot, aphids, Japanese beetles and sooty mold. More information on successfully growing crape myrtles is available in HGIC 1008, Crape Myrtle, and HGIC 1009, Crape Myrtle Pruning.

Diseases

Powdery Mildew: Powdery mildew is one of the most common problems of crape myrtle, and it is caused by the fungus Erysiphe lagerstroemia. Patches of white to grayish powdery growth occur on the surfaces of leaves, flowers and new shoots. Heavily infected flowers may fail to open. Infected parts of the plant are usually distorted and stunted. The disease is most serious in shady, damp locations, especially where plants are crowded and air circulation is poor. Development of the fungus is favored by high humidity at night and dry, mild daytime conditions, as often occurs during the spring and fall.

Larger image (148 Kb)
Powdery mildew coating the flower buds of crape myrtle.
James Blake, ©2007 HGIC, Clemson Extension

Prevention and Treatment: The most effective control measures include locating plants in full sun, removing sprouts from the base of the plant and planting resistant varieties. Susceptible varieties of crape myrtle should be avoided. Removing diseased twigs and branches may be possible, if only a few shoots are infected. Remove sprouts (suckers) at the base of the plant as they occur, since they are very susceptible to powdery mildew. Once these sprouts become infected, the fungus easily spreads to the upper portions of the plant.

Plant Resistant Varieties: The extent of resistance to powdery mildew for a particular variety may vary from location to location and may depend on particular conditions occurring in the environment.

• The Lagerstroemia indica x fauriei hybrids developed at the U. S. National Arboretum in Washington, D. C. are resistant to powdery mildew.
• Varieties with good resistance to powdery mildew and fairly good tolerance for Cercospora leaf spot include: ‘Apalachee,’ ‘Basham’s Party Pink,’ ‘Caddo,’ ‘Dodd #2’ ‘ Fantasy,’‘Glendora White,’ ‘Hopi,’ ‘Lipan,’ ‘Miami,’ ‘ Osage,’ ‘Pecos,’ ‘Regal Red,’ ‘Sarah’s Favorite,’ ‘Tonto,’ ‘Tuscarora,’ ‘Tuskegee,’ ‘Velma’s Royal Delight’ and ‘Wichita. ’  ‘Apalachee’ and ‘Fantasy’ are totally resistant to powdery mildew.
• ‘Catawba,’ ‘Cherokee’ and ‘Seminole’ have some resistance to powdery mildew.

Varieties to Avoid: ‘Gray’s Red,’ ‘Orbin’s Adkins, ’ ‘Carolina Beauty,’ ‘Wonderful White,’ ‘Raspberry Sundae’ and ‘ Potomac. ’

If disease is severe enough to warrant chemical control, select a fungicide containing one of the following: propiconazole (Ferti-loame Liquid Systemic Fungicide), thiophanate-methyl (Ferti-loame Halt Systemic Fungicide or Green Light Systemic Fungicide), triadimefon (Green Light Fung-Away), triforine (Ortho Orthenex Insect & Disease Control), copper-based fungicides (Ferti-lome Black Spot & Powdery Mildew Control or Concern Copper Soap Fungicide RTU or Safer Garden Fungicide RTU) , or lime sulfur (Hi-Yield Lime Sulphur Spray). Multiple applications may be required. Triadimefon can cause some foliar discoloration and distortion on roses. Apply all chemicals according to directions indicated on the label.

Cercospora Leaf Spot: Leaf spots may appear on crape myrtle that are caused by the fungus Cercospora species during periods of warm, moist weather. Yellow spots (1/8 to ¼ inch diameter) appear on the upper leaf surface with white-grey sporulation of the fungus on the lower leaf surface. The disease can result in almost complete defoliation of the plant in late summer and fall in susceptible cultivars.

Prevention and Treatment: Select resistant varieties for new plantings. The varieties, ‘Fantasy,’ ‘ Tonto,’ ‘Tuscarora,’ ‘Tuskegee’ and ‘ Velma’s Royal Delight,’ have exhibited resistance to Cercospora leaf spot in field trials. The amount of resistance may vary from location to location and may depend on particular environmental conditions. Provide good air circulation and avoid overcrowding plants. If disease is severe enough to warrant using chemicals for control, thiophanate-methyl as used for powdery mildew (see above) will control Cercospora leaf spot. Apply all chemicals according to directions on the label.

Insects and Other Pests

Very few insects are pests of crape myrtle. In South Carolina, the most important insect  pest is the crape myrtle aphid (Tinocallis kahawaluokalani), which is found on the leaves and twigs of crape myrtle. Crape myrtle aphids feed only on crape myrtle trees. Japanese beetle (Popillia japonica) is also a pest, feeding on leaves and flowers. In addition to crape myrtle, it will feed on nearly three hundred different plant species.

Crape Myrtle Aphids: Crape myrtle aphids are pale yellowish green in color with black spots on the abdomen. They vary in length from 1/16 to 1/8 inch long. They overwinter (survive the winter) as eggs. The eggs hatch in the spring. During the growing season, females give birth to live young. Since it takes about 10 days to reach maturity, several generations are produced each growing season. At the end of the growing season, females produce eggs that overwinter.

Aphids feed by inserting their mouthparts into tender new leaves from which they suck plant sap. Plant sap has a high sugar content. When they feed, the aphids excrete large amounts of a sugary liquid called honeydew. With a large aphid population, the honeydew can completely coat leaves. The honeydew serves as food for the sooty mold fungi, as well as various insects, including ants, wasps and flies.

Larger image (57.3 Kb)
A heavy infestation of aphids on the underside of a leaf.
Image from John Herbert, University of Florida

As the aphid feeds, it injects saliva into the leaf. The saliva causes yellow spots to develop on the leaf. Their feeding on young leaves often causes distortion of leaves. Buds, branch tips and flowers can also be affected by feeding.

Control: The following crape myrtle hybrids (Lagerstroemia indica x fauriei) have moderate resistance to aphids: ‘Muskogee,’ ‘Natchez,’ ‘Tuscarora, ’ ‘Acoma,’ ‘Tuskegee,’ ‘Hopi,’ ‘Pecos,’ ‘Zuni,’ ‘Biloxi,’ ‘Miami, ’ ‘Wichita,’ ‘Apalache,’ ‘Comanche,’ ‘Lipan,’ ‘Osage,’ ‘Sioux,’ ‘Yuma,’ ‘Caddo,’ ‘Tonto, ’ ‘Choctaw’ and ‘ Fantasy.’  Consider using these in new plantings.

Several predators feed on the crape myrtle aphid. These include ladybird beetles (ladybugs) and their larvae (immature forms), green lacewings and their larvae, hover fly maggots, parasitic wasps and entomophagous (insect feeding) fungi. As much as possible, these natural predators should be allowed to reduce aphid populations. In addition, aphids can sometimes be removed from plants by spraying with a strong stream of water. Spraying with water may have to be repeated regularly, as needed.

As a result of their phenomenal reproductive rate, aphids are very difficult to control with insecticides. If a single aphid survives, a new colony can be produced in a short period of time. In addition, using insecticides means that beneficial predators will also be killed. If it is determined to be absolutely necessary, various insecticides are labeled for use by homeowners against aphids on crape myrtles. These include insecticidal soap (Safer Insecticidal Soap or Concern Insect Killing Soap), horticultural oil (Sunspray Ultra-fine Spreay Oil), pyrethrins (Schultz Rose & Flower Insect Spray or Concern Multi-Purpose Insect Killer RTU), neem oil (Bonide Bon-Neem or Green Light Neem Concentrate) or permethrin (Spectracide Bug Stop Multi-Purpose Insect Control Concentrate) or cyfluthrin (Bayer Advanced Garden Power Force Multi-Insect Killer Concentrate) or malathion. As with all pesticides, read and follow all label instructions and precautions.

Japanese Beetles: Adult Japanese beetles are about ½ inch in length and coppery-brown in color with metallic green heads. They emerge from the soil and feed from May to August. They lay their eggs in the soil. Grubs hatch from the eggs and feed on grass roots. As the weather cools, the grubs move more deeply into the soil, to overwinter.

Both adult beetles and their larvae (grubs) can seriously damage plants as a result of their feeding. Adult Japanese beetles eat flowers and skeletonize leaves (eat leaf tissue between the veins, resulting in a lacy skeleton remaining). The grubs feed on the roots of plants, especially on the roots of grasses.

Control: Multiple approaches are necessary for controlling Japanese beetles. Adults can be removed by handpicking and destroyed. Japanese beetle traps are available commercially or can be homemade. They may be effective at reducing adult populations.

Keep traps at least 50 feet from the crape myrtle tree or you may create more of a problem by attracting them to the area. Milky spore, Bacillus popilliae, is a disease-causing bacterium that is effective against grubs of Japanese beetles but not the adults. It is commercially available for homeowner use. Carbaryl (Sevin 50WP) or neem oil (Bonide Bon-Neem or Green Light Neem Concentrate) or cyfluthrin (Bayer Advanced Garden Power Force Multi-Insect Killer Concentrate) or permethrin (Spectracide Bug Stop Multi-Purpose Insect Control Concentrate) are labeled for use by homeowners against Japanese beetles on crape myrtle. As with all pesticides, read and follow all label instructions and precautions.

Other Problems

Sooty Mold: Leaf and stem surfaces are covered with a black sooty substance, causing them to appear black and dirty. Sooty mold indicates that there is an insect problem on the plant. These common molds are caused by fungi that grow on the sugary substance, called honeydew, produced by various insects that suck sap from the plant. Aphids, scales, mealybugs and whiteflies most commonly cause this problem.

Larger image (70.3 Kb)
Sooty mold completely covers these crape myrtle leaves.
Image from John Herbert, University of Florida

Prevention and Treatment: Sooty molds are unsightly, but are relatively harmless, since they do not directly attack the plant. Controlling the insect problem can reduce excessive amounts of sooty mold. Reduce aphid numbers by allowing beneficial insects such as lady beetles to inhabit the plant. Aphids can sometimes be washed from the plant with a strong stream of water.

Although not necessary, sooty mold can be washed from the leaves by drenching with detergent (4 ounces per gallon of water), waiting three to four minutes, then washing with a strong stream of water.

Lichens: A lichen is an unusual organism composed of a fungus and an alga living together in the same body. Lichens often appear as green to gray-green leafy or crusty growths on the trunks or branches of plants. Typically, they occur in abundance on plants that are declining in health or vigor. They are harmless to the plant, and are in no way responsible for the poor health of the plant. Less vigorous plants tend to be more open, increasing sunlight penetration, and subsequent lichen growth.

Larger image (133 K)
These harmless lichens are growing on an older crape myrtle.
Karen Russ, ©2007 HGIC, Clemson Extension

Prevention and Treatment: Controls are not necessary. Lichens will gradually disappear if the health of the plant is restored.

Note: Control of diseases and insects on large trees is usually not feasible, since adequate coverage of the foliage with a pesticide cannot be achieved.

The images of crape myrtle aphids and sooty mold images are courtesy of the University of Florida. They are originally from the UFL publication EENY-365 (IN663) Crapemyrtle Aphid, Sarucallis kahawaluokalani (Kirkaldy) (Insecta: Hemiptera: Aphididae). Published: February, 2006