The dark, unsightly coating that appears on the bark and leaves of a crepe myrtle is not a direct disease of the tree itself, but rather a fungal growth known as Sooty Mold. This mold is sustained by a sticky, sweet substance excreted by sap-sucking insects feeding on the plant’s tissues. The fungus is non-parasitic and grows only on this sugary waste product, called honeydew. Addressing the black residue requires a dual approach: physically removing the existing mold and, more importantly, eliminating the insect infestation that provides the food source. This guide provides a complete strategy for diagnosing the cause and implementing effective treatment.
Understanding the Source of Black Bark
The presence of Sooty Mold signals an active infestation of sap-sucking insects that are poorly digesting the plant’s phloem sap and excreting the excess sugar as honeydew. Two primary pests are responsible for this issue on crepe myrtles: the Crepe Myrtle Bark Scale (CMBS) and Crepe Myrtle Aphids. The black mold itself does not attack the tree’s living tissue, but a heavy coating on the leaves can inhibit photosynthesis by blocking sunlight.
Crepe Myrtle Aphids are small, pale yellowish-green insects typically found feeding on the undersides of new leaves and tender growth during the spring and summer months. These pests produce copious amounts of honeydew that drip down onto the stems, bark, and surrounding surfaces. A more serious culprit is the Crepe Myrtle Bark Scale, which appears as tiny, white or gray, felt-like bumps on the trunks and branches, often congregating in branch crotches or around old pruning scars.
When the black mold is concentrated on the trunks and woody branches, CMBS is the likely cause, while mold on the leaves often points to an aphid problem. Proper identification of the insect is a necessary first step, as treatment methods vary between the soft-bodied aphids and the armored scale.
Physically Removing the Black Mold
Once the underlying insect problem is identified and addressed, the physical removal of the Sooty Mold improves the tree’s appearance and overall health. The crepe myrtle’s naturally thin, smooth bark requires a gentle cleaning approach to prevent any abrasive damage.
A mild solution of diluted insecticidal soap or even a few drops of mild dish soap mixed into a gallon of water works effectively as a cleaning agent. Using a soft-bristled brush, sponge, or rag, gently scrub the affected areas of the bark, working the solution into the black residue. Avoid harsh cleaners or high-pressure washing, which can strip or damage the delicate bark tissue.
After scrubbing, the bark must be rinsed thoroughly with a strong stream of water from a garden hose to remove all soap and mold debris. The soap solution helps break down the sticky honeydew film, allowing the water to wash away the fungal spores and pigment. While cleaning the mold offers immediate aesthetic improvement, the blackening will return quickly if the sap-sucking insects are not completely eradicated.
Controlling the Sap-Sucking Pests
Eliminating the insects that produce honeydew is the only way to permanently resolve the Sooty Mold issue. Control strategies must be tailored, as aphids are generally easier to manage than the more persistent Crepe Myrtle Bark Scale. For aphids, which are soft-bodied, a strong jet of water can physically dislodge them from the leaves, or they can be effectively treated with topical sprays.
Topical applications of horticultural oil or insecticidal soap are effective against both aphids and the mobile crawler stage of scale insects. These treatments work by suffocating the pests, so complete coverage of all infested areas, including the undersides of leaves, is required for success. Horticultural oils are generally applied during the dormant season or as a lighter summer oil, always following product-specific instructions to prevent leaf burn.
For severe infestations, particularly with Crepe Myrtle Bark Scale, systemic insecticides offer the most reliable control, as the chemical moves throughout the tree’s vascular system. Products containing active ingredients like imidacloprid or dinotefuran are applied as a soil drench around the base of the tree. The optimal timing for a systemic application is in the early spring, typically from late March through May, as the tree begins to absorb water and nutrients.
Another effective systemic method is painting a concentrated paste of an acephate product directly onto the bark of the trunk, where the thin bark allows for absorption. Encouraging natural predators, such as lady beetles, can also help manage aphid populations, though they may not control a heavy scale infestation alone.
Preventing Future Outbreaks
Long-term management relies on cultural practices that create an environment less favorable to the pests and the subsequent mold. Proper pruning is important, as thinning the canopy increases air circulation and light penetration, helping to dry the bark and inhibit fungal growth. Removing overly dense growth reduces the humid, sheltered conditions that Sooty Mold thrives in.
Careful management of fertilizer is also a preventive measure. High-nitrogen fertilizers promote a flush of soft, new growth that is particularly attractive to sap-sucking insects like aphids. Using a balanced or slow-release fertilizer, or avoiding excessive fertilization altogether, helps limit the quantity of this susceptible plant material.
Regularly inspecting the trees, especially during peak growing seasons, is the most effective preventative action. Early detection of small clusters of aphids or the first signs of scale allows for targeted, less aggressive treatments before a full-blown infestation develops. This proactive monitoring minimizes the chance of honeydew production and the subsequent return of the black mold.