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

New scale attacks stems and twigs of crape myrtle.

Adult ladybugs attach themselves to scale insects as they devour them.

Progression of the beneficial ladybugs from nymphs with black, spiny legs, into shiny black-winged adults with matching orange spots on wings – everything you see in photo is beneficial.

Since 2005, outbreaks of a new and as-yet unidentified scale insect have been seen on crape myrtles. Only a few examples of the pest were reported in 2005, predominantly from the north suburbs of Dallas, in southern Collin County. A few more examples were reported in 2006. However, populations swelled in the spring and summer of 2007. May and June were cool and very wet, and many types of scale insects were reported in large populations, this newcomer to crape myrtles included.  Soon, the pests were reported over much of northern Dallas and most of Collin and Denton Counties. In ensuing years, they have appeared elsewhere across Texas and southern Oklahoma.

The first thing that homeowners see is a rapid build-up of mealy-bug like insects, actually resembling a plastering of cigarette ashes on stems and trunks. When the pests are squeezed with a twig or thumbnail, a bright red fluid exudes.

Soon after the appearance of the scales in 2007, an uncommon ladybug started to proliferate as well. It is a very shiny black insect with two matching bright orange spots on its wings. Immature stages of the beneficial ladybugs, as shown in the photo, have many small black and spiny-looking legs. That stage soon evolves into the adult ladybugs. These ladybugs quickly proved to be very effective in eliminating the population of scale insects. After only a week or two, the scales were almost entirely devoured. When what was left of their bodies was pressed, no red fluid exuded. The ladybugs had consumed a larger percentage of the scales.

The Crape Myrtle Trails of McKinney asked for help from urban entomologist Dr. Mike Merchant of the Texas AgriLife Extension in Dallas. In 2008, Dr. Merchant and his team applied a series of scale controls along Eldorado Parkway in McKinney. While they were able to get some initial indications of the best means of controlling the scale, populations in 2008 and after have not been sufficient to accomplish good testing.

At this time, it appears that a horticultural oil spray applied in the winter is a good means of reducing populations of the overwintering scale insects. Systemic insecticides offer hope in controlling ongoing outbreaks. However, home gardeners should check their plants closely for evidence of the appearance of the beneficial black ladybug. Since they are so efficient in reducing the populations of the scale, if ladybug nymphs or adults are present, spraying probably should be put on hold. More tests will continue as needed.

Homeowners are best advised to watch their plants closely, especially during spring and summer. Should the scales appear, act quickly. Left unchecked, the scale insects will leave a sticky honeydew residue on the trunks, and black sooty mold will develop in that residue. While not harmful to the plants, it is certainly not attractive. It will eventually disappear as the bark sloughs off in the following year or two. The way to control or prevent it is to intervene in the development of the scale in the first place.