Powdery Mildew on Crape Myrtles

Powdery mildew on crape myrtle leaves, twigs

Compiled from report of Dr. Kevin Ong
Plant Pathologist
Texas Cooperative Extension

Powdery mildew (Erysiphe lagerstroemiae) is a fungal disease attacking crape myrtle leaves, shoots, buds and flowers. It produces numerous spores which, when seen under a microscope, resemble chains of beads. The result is a powder-like dusting that grows in thin layers on the plant tissue surfaces.

Powdery mildew is most evident on young, actively growing plant tissues. It can cover new growth entirely in late spring and into early summer and cause severe damage to those plant tissues, causing leaves to become misshapen and even delaying the first summer bloom until the disease abates. Research scientists comparing variations in varietal susceptibility to mildew generally look most closely at the first flush of flower buds in early summer, as that is where it often begins.

Warm days and cool nights (60F-80F) favor the growth and development of the fungus. Its spores are disseminated by wind. Infection can occur in as few as 3 to 5 days after the spores land on their host. The spores can germinate and infect the crape myrtle in the absence of free water on the plant tissues as long as there is adequate humidity in the air.

The best management practice for powdery mildew is prevention from the outset. Choose resistant varieties. Plant crape myrtles in full sun and where there will be good air movement to keep the leaf surfaces as dry as possible. Avoid excessive amounts of fertilizer that might promote soft growth that could be unnecessarily susceptible to the fungus. Natural fungicides such as Neem oil extract and potassium bicarbonate have been shown to be effective when used correctly. Several good synthetic fungicides also offer good control. Treatment will normally not be needed once temperatures climb into the 90s on a daily basis, as the disease becomes relatively inactive at those temperatures.

Crape Myrtle Insects

Kimberly Engler
Program Specialist-Urban IPM
Texas Cooperative Extension

Crape myrtles are very popular plants among Southern gardeners due to their beauty and low maintenance requirements. Planting and maintaining crape myrtles according to recommended procedures will give them the best possible chance at warding off pests. A stressed crape myrtle is more susceptible to damage by insects and diseases.

The major insect pests of crape myrtles in Texas are crape myrtle aphids, Japanese beetles, and primrose flea beetles. A few insects on a crape myrtle usually do not cause significant damage to the plant. However, treatment may be necessary to prevent further damage if an insect population increases. Treatment options may include both non-chemical and chemical practices. Chemical insecticide application should be used as a last resort and, wherever possible, the least toxic, most target-specific insecticide should be applied. 

Crape myrtle aphids secrete sticky honeydew from mid-summer on. Sooty mold grows in the honeydew.
(click to enlarge)

Crape myrtle aphid (Tinocallis kahawaluokalani Kirkaldy) has a narrow host range and is the only aphid species that feeds on crape myrtles. Crape myrtle aphids are found from May through September, with peak populations during July and early August. Crape myrtle aphid adults and nymphs are pale yellow-green with black spots on their abdomens. They range from 1/16- to 1/8-inch in length. These aphids generally feed on the undersides of crape myrtle leaves. They feed by inserting their needle-like mouthparts into the soft tissues and extracting plant sap. As aphids feed, they inject saliva into the leaf tissues, which, in turn, causes yellow leaf spots to develop. Heavy infestations can deform leaves and stunt new growth. Buds, branch tips and flowers can also be damaged by the feeding injury. 

In addition to direct damage, aphids can damage plants indirectly by secreting honeydew, a sugary bi-product of their feeding. Honeydew is a perfect substrate for the growth of sooty mold fungi and a source of food for other insects such as ants, wasps and flies. Although unsightly, sooty mold itself does not directly harm the crape myrtle. Instead, it shades the leaves and interferes with photosynthesis. Under heavy infestations, sooty-mold-covered leaves may drop many weeks prematurely. Even the twigs and stems of the plants may be coated in the black sooty mold. That becomes more evident as the plants defoliate.

Japanese beetle (Popillia japonica) Newman, was introduced into New Jersey in 1916. It spread throughout the eastern U.S. and then into the southern states. It is 1/3-inch in length and broad oval in shape. Its wings are coppery in color with fine longitudinal lines, and its body is a beautiful metallic green. It has five tufts of white hairs projecting from under the front wing on each side, with two patches of white hairs at the tip of the abdomen.

Both adults and larvae can damage crape myrtles. The larvae live underground where they usually feed on roots of grasses, but may consume young tree roots. The adults prefer to eat the tissue between the leaf veins, causing a lacelike appearance. Adults are most active during the warmest part of the day and are usually found on plants that are in full sun. 

Primrose flea beetle (Altica litigata) Fall, is quite small, to 1/8-in in length, oval in shape, and shiny blue-green in color. All the immature lifestages of a flea beetle occur underground, so only the adults are seen by gardeners. The adult beetles lay their eggs and the larvae feed on the roots of various plants in the primrose family. However, the larvae can feed on the foliage or tunnel into the plant stems. Adults feed on crape myrtle, evening primrose and other plants, although the normal host is Ludwigia.  The adult beetles will chew many holes or pits into the leaves, leaving "shot holes" in the leaves. The effect of their feeding is most severe when they attack the growing tips because this limits the ability of the plant to compensate for damage making young plants and seedlings particularly susceptible. As a result, growth may be reduced or the plant may die.

A New Pest of Crape Myrtles