The crepe myrtle (Lagerstroemia indica) is a popular ornamental tree known for its vibrant summer blooms and distinctive bark. A white coating on the tree often signals a potential health issue. Identifying this substance is the first step toward effective treatment, as the coating indicates one of two very different problems requiring separate solutions. Understanding the underlying cause allows for targeted action to protect the tree and restore its aesthetic value.
Differentiating the White Coating
The white coating is typically one of two possibilities: powdery mildew (a fungal disease) or Crepe Myrtle Bark Scale (CMBS), an insect infestation. Powdery mildew (Erysiphe lagerstroemiae) appears as a fine, dusty, white-to-gray layer, usually on new growth, leaves, and flower buds. This mildew thrives in high humidity, especially when warm days follow cool nights, and can cause leaves to twist, distort, or drop prematurely.
Crepe Myrtle Bark Scale is an insect pest that manifests as clusters of white or gray, felt-like, waxy bumps on the bark of trunks and branches. These insects feed on the tree’s sap, excreting a sugary substance called honeydew. The honeydew quickly leads to the development of sooty mold, a separate black fungus that grows on the sticky residue, often covering the trunk and nearby surfaces. Scale is found on woody parts of the tree, while powdery mildew is primarily on the foliage and new shoots. Crushing an active scale insect will also produce a diagnostic pink liquid.
Immediate Solutions for Removal
The removal method depends entirely on the correct diagnosis; a fungicide will not treat an insect, and an insecticide will not treat a fungus.
Powdery Mildew Treatment
For light infections, physical removal is possible by pruning off severely infected tips and shoots. Applying a fungicidal spray is the next step. Options include organic treatments like Neem oil or horticultural oils, which have protective and limited eradicant properties. For more established mildew, synthetic fungicides containing active ingredients like myclobutanil or propiconazole offer a more aggressive solution. Applications must thoroughly coat both the top and bottom of affected leaves.
Crepe Myrtle Bark Scale Treatment
Treating CMBS requires eliminating the insect to stop the production of honeydew and the resulting sooty mold. Heavily infested small branches should be pruned and removed immediately to eliminate the scale population. Scale present on the trunk and large branches can be physically removed using a gentle scrub with a soft-bristled brush and a solution of mild detergent and water, which is effective against sooty mold. Chemical control of CMBS is most effective using systemic insecticides containing ingredients such as imidacloprid or dinotefuran. These are applied as a soil drench in late winter or early spring. The tree absorbs the systemic insecticide through the roots, offering season-long control. A dormant oil application in late fall or early spring, before new growth appears, can also smother overwintering scale insects and eggs.
Long-Term Cultural Practices for Prevention
Preventing recurrence of either powdery mildew or scale involves improving the crepe myrtle’s growing environment and overall health. Proper planting location is necessary, as crepe myrtles require full sun exposure to discourage the humid, shaded conditions that favor powdery mildew. Trees planted in shade are also less vigorous and more susceptible to pests.
Thinning the tree’s canopy through selective pruning in late winter is a preventive measure that improves air circulation within the branches. Increased airflow helps dry leaf surfaces quickly, making it difficult for powdery mildew spores to germinate and spread. Avoiding overhead watering, especially late in the day, helps keep the foliage dry and reduces the environment conducive to fungal growth. Homeowners can also select cultivars that exhibit natural resistance to powdery mildew, such as ‘Muskogee’ or ‘Natchez.’ Maintaining a healthy tree through deep, infrequent watering and avoiding excessive fertilization will boost the tree’s natural defenses.