Crepe (or crape) myrtles (Lagerstroemia indica) are deciduous trees, hardy to U.S. Department of Agriculture plant hardiness zones 7 through 9. The tree is beloved for its showy flower panicles that bloom from mid-summer through fall. Some crepe myrtle cultivars, such as “Apalachee” (Lagerstroemia indica “Apalachee”) and “Biloxi” (Lagerstroemia indica “Biloxi” also offer interesting peeling bark on their trunks. Because the tree is so highly ornamental, owners grow understandably alarmed if the tree’s foliage turns black.
Many plant disorders cause leaves to change color, but few cause them to turn black. If the leaves still retain some green coloring and the black portions are mainly along the edges, the crepe myrtle may be suffering from leaf scorch which is not a true disease but is caused by improper watering. When the entire plant turns black and appears to be dirty, suspect a pest infestation.
The black, sooty substance on the crepe myrtle, known as sooty mold, is caused by fungal pathogens that are not attacking the plant, but feasting on a sticky substance excreted on the leaves by certain common garden pests. Insects that are fond of crape myrtle and that excrete honeydew include scales (Eriococcus lagerostroemia), greenhouse whiteflies (Trialeurodes vaporariorum), crepe myrtle aphids (Tinocallis kahawaluokalani) and mealybugs (Pseudococcidae family). Sooty mold then is a symptom of a pest infestation and not a disease of the plant. If the mold is thick enough it may block sunlight from reaching the foliage, leaving it unable to carry out photosynthesis, so controlling the pest population is imperative.
Control Scales, Aphids and Mealybugs
When the pest infestation is controlled, sooty mold goes away. Aphids are the easiest to control – just use a blast of water from the hose to knock them off the plant. Scales, crepe myrtle aphids and mealybugs can also be controlled with a cottonseed-based horticultural oil spray. Use 3 ounces of the product in 1 gallon of water in a tank or backpack sprayer and cover all parts of the crepe myrtle with the spray.
Whiteflies are the most difficult to control and require a three-pronged approach. Use a systemic insecticide with the active ingredient acetamiprid. Use 1.5 ounces of insecticide in 1 gallon of water in a tank sprayer and spray the upper and lower portions of the crape myrtle’s foliage until they are wet but not dripping. One month later, apply an insecticide with the active ingredient imidacloprid at a rate of ½ ounce per inch of trunk diameter, at chest height. If the crepe myrtle has multiple trunks, measure the diameter of each one, add them up and multiply that number by 0.375. The result is the number of ounces of insecticide to apply. Add the insecticide to 1 gallon of water and pour the solution around the crepe myrtle, near the trunk. Follow that up with one gallon of clean water, applied over the same area that you applied the insecticide. If the pests persist after several months, spray the crepe myrtle with the oil recommended for scale, aphids and mealybugs, but use 6 ounces in 1 gallon of water.