The Crape Myrtle, Lagerstroemia indica, is a highly valued ornamental tree celebrated for its smooth bark and long summer bloom period. When a noticeable white substance appears, it signifies a problem requiring attention. This visual change is typically caused by either an insect infestation or a fungal disease. Identifying the source is the first step toward resolving the issue and restoring the tree’s appearance.
Identifying the Culprit: Scale Insect versus Fungus
The white material is typically caused by either the Crape Myrtle Bark Scale (CMBS) insect or Powdery Mildew, a fungal disease. These two problems are easily distinguishable by location and texture. CMBS appears as waxy, felt-like, or cottony white masses concentrated on the smooth bark of the trunk, branches, and especially in branch crotches. Crushing these masses may exude a pink liquid, which is a diagnostic sign of the scale insect. Powdery Mildew, in contrast, presents as a fine, fuzzy, flour-like coating. It is most often found on the leaves, new shoot growth, and flower buds. This coating is the visible fungal growth and can be easily wiped away with a finger.
Crape Myrtle Bark Scale (CMBS): Damage and Treatment
Crape Myrtle Bark Scale (Acanthococcus lagerstroemiae) is an invasive insect that weakens the tree by feeding on its sap. While CMBS rarely kills a mature tree, heavy infestations reduce plant vigor, delay spring leafing, and decrease the size and abundance of flower clusters.
As the insects feed, they excrete a sugary waste product called honeydew, which coats the bark and leaves. A black fungus, known as sooty mold, then grows over this sticky honeydew, creating an unsightly black coating. The presence of sooty mold is often the first symptom noticed, and it is a secondary issue caused by the insect infestation.
For a light infestation, physical removal of the scale and sooty mold is an immediate treatment. Scrub the affected bark areas with a soft brush and a solution of mild dish soap and warm water. This mechanical cleaning reduces the initial population and removes the sooty mold, allowing the bark to photosynthesize.
For more severe infestations, chemical control is necessary because the scale’s waxy coating protects them from contact sprays. The most effective approach involves systemic insecticides, such as products containing dinotefuran or imidacloprid, applied to the soil around the root zone. The tree absorbs these chemicals through its roots, which then move throughout the plant system to kill the feeding insects.
Systemic treatments are applied in late winter or early spring before the tree fully leafs out. This allows the product time to circulate before the scale population peaks in summer. Dormant horticultural oils can also be applied to the bark during the winter to smother overwintering adults and eggs. All insecticide use must follow label directions precisely to minimize potential harm to beneficial insects.
Powdery Mildew: Causes and Management
Powdery Mildew is caused by various species of fungus, such as Erysiphe lagerstroemiae, and is a common disease affecting Crape Myrtles. This fungal disease thrives in specific environmental conditions: high humidity at night combined with dry, mild temperatures during the day. It often affects the newest, most tender growth, resulting in stunted, distorted leaves and flower buds that may fail to open.
While the disease is rarely fatal, it can significantly detract from the tree’s aesthetic value and stunt overall growth. Management begins with cultural practices designed to make the environment less favorable for the fungus. Planting Crape Myrtles in full sun is a primary preventative measure, as the spores are sensitive to heat and sunlight.
Pruning techniques that improve air circulation within the canopy are also effective at reducing humidity and allowing foliage to dry more quickly. This includes removing suckers at the base of the plant, as their new growth is susceptible to infection. For established plants with a history of severe infection, fungicides can be used, but they are most effective when applied preventatively, before the disease appears.
Products containing active ingredients like myclobutanil or thiophanate-methyl are available for chemical control. Less toxic treatments, such as horticultural oil or neem oil, can also act as contact treatments against the fungal growth. Repeat applications, typically every seven to ten days, may be necessary to protect new growth.
Long-Term Health and Prevention
Preventative measures are the most sustainable approach for maintaining a healthy Crape Myrtle and reducing its susceptibility to both scale and mildew. The correct planting location is paramount, as these trees thrive best in areas that receive full sun and have well-draining soil. Adequate sunlight maximizes the tree’s natural defenses against mildew and promotes vigorous growth.
Strategic pruning is another long-term health practice that focuses on thinning interior branches to maximize air flow throughout the canopy. This helps discourage the high-humidity conditions that favor Powdery Mildew development. Avoiding excessive nitrogen fertilization is also beneficial, as this encourages the flush of tender new growth vulnerable to both the scale insect and the mildew fungus.
One of the most effective preventative strategies is selecting cultivars bred for resistance to these common problems. Many modern hybrid varieties, particularly those developed by the U.S. National Arboretum, exhibit good to excellent resistance to Powdery Mildew. Resistant cultivars include ‘Natchez’ (white), ‘Tuscarora’ (coral pink), and ‘Muskogee’ (lavender pink). While no commercially available Crape Myrtle is fully resistant to CMBS, choosing a mildew-resistant variety reduces the overall burden on the tree, contributing to its long-term health.