Powdery Mildew on Crape Myrtles
Compiled from report of
Dr. Kevin Ong,
Director of Texas A&M
Plant Disease Diagnostic Laboratory
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.
Former Program Specialists-
Urban IPM Texas
AgriLife Extension Service
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 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
Update on Scale Appearing on North Texas Crape Myrtles
The Crape Myrtle Trails of McKinney horticulture committee has asked the help of Texas AgriLife Extension Entomologist Dr. Mike Merchant in identifying a relatively new insect pest on crape myrtles, as well as determining the nature of any damage done by it, and the best means of addressing the pest in the landscape. Here is his report.
Update on Crape Myrtle Bark Scale
Dr. Mike Merchant,
Professor and Extension Urban Entomologist, Texas AgriLife Extension Service,
Dallas, May 11, 2012
In 2004 a report was received in our office from a local landscape maintenance company concerning an unusual pest insect on crape myrtle that was proving exceptionally difficult to control. This initial infestation was located on a commercial property in Richardson, TX near the intersection of Jupiter and Renner Roads. Initially this insect was identified as an Eriococcid scale (Hemiptera: Eriococcidae), most likely the azalea bark scale, Eriococcus azaleae.
The scale infestation we observed at this first site was very heavy, with scales covering the upper branches of the infested crape myrtle trees, and trunks coated with a heavy layer of black sooty mold. Since this time reports of the scale have been received in our office, first primarily from the north Dallas to Richardson area, then from a wider region surrounding the Dallas]Fort Worth Metroplex. Most recently we have received reports of the scale in Denton, TX and Ardmore, OK. To our knowledge, the scale is not established in areas outside north Texas or southern Oklahoma.
Because of the fact that azalea bark scale had never before been recorded from crape myrtle, and our learning of a very similar species of eriococcid scale that feeds on crape myrtle from China and Japan, we resubmitted scale samples to Dr. Dug Miller in 2010, systematic taxonomist for scale insects with the USDA in Washington, DC. He concluded that it was very likely that this scale insect is a new introduction of Eriococcus lagerostroemia, the scale previously known only from Asia. Because he is unable to detect any physical differences between our crape myrtle scale and the azalea bark scale, we have concluded that DNA analysis will be necessary to differentiate the two species.
In 2008 our laboratory received funding from the Texas Nursery and Landscape Association to evaluate different insecticide treatments for this scale. In addition to testing some newer neonicotinoid insecticides, we looked at applications of horticultural oil and of malathion. The latter two treatments provided no significant suppression of scale activity; however we did see significant control with acetamiprid, clothianidan, dinotefuran and imidacloprid, all commercially available insecticides. Two of these products, dinotefuran and imidacloprid, are available to consumers. When applied with a drench treatment to the plant root zone, we concluded that these products provide significant control of this scale, although additional work needs to be done concerning the longevity of control and consumer satisfaction associated with use of these products.
Current recommendations for control of this scale can be found online at http://citybugs.tamu.edu/2010/05/10/scale These recommendations include:
♦ For heavily infested plants wash the trunk and reachable limbs with a soft brush and mild solution of dishwashing soap. This will remove many of the female scales and egg masses and make insecticide control more effective. Also, washing will remove much of the black mold that builds up on the bark on infested trees.
♦ Horticultural oil has not yet been shown to be effective against this insect, however a winter application of dormant oil to the bark and crotches of the plants where scales shelter may be beneficial. Winter is an especially good time to treat for scales because a higher (winter) application rate can be used without damaging the plant. Thorough coverage of the tree is especially important when treating with oil.
♦ Application of systemic insecticides as a drench applied to the root zone of plants to be protected has shown the most promise in tests to date. Imidacloprid (Merit® or Bayer AdvancedTM Garden Tree and Shrub Insect Control) and dinotefuran (Greenlight Tree and Shrub Insect Control with Safari) has shown best control when applied between May and July. When drenching the soil with a systemic insecticide, allow several weeks for control as the products needs time to spread throughout the plant.
It should be noted that we have not received any significant complaints from the public about tree death as a result of infestation with this scale. It is my opinion that while the scale is capable of stressing these trees, it does not usually kill otherwise healthy crape myrtles. Some years the scale does not appear to be very heavy in many locations, likely due to biological control from lady beetles and other predatory insects. In other years, and in certain locations, the scale can detract significantly from the appearance of crape myrtle plantings.
There is much still to learn about the identity and management of this new pest. Recommendations for future research include:
♦ Is this insect indeed, E. lagerostroemia, or another species of Eriococcus?
♦ How long will one application of the various systemic insecticides provide satisfactory scale control?
♦ Are there additional, low impact insecticides that will provide adequate control?
♦ What beneficial insects are associated with control of this scale, and are there strategies that will preserve these insects?
♦ What are costs associated with control of the crape myrtle bark scale? Can control costs be reduced?
♦ What is the biology of this pest in Texas? How many generations does it have, and what are the environmental and ecological factors responsible for annual variations in its abundance.
♦ How far and how fast will this scale spread in the U.S.?