A black, powdery residue on the leaves and bark of a crepe myrtle is commonly called “black fungus,” but it is correctly identified as sooty mold. This superficial fungus thrives on the tree’s surface. While the mold is generally not fatal, it is unsightly and can impair the tree’s health by blocking sunlight, which hinders photosynthesis. The presence of this mold is a clear visual symptom of an underlying insect problem. Addressing this condition involves a two-part approach: physically removing the existing mold and, more importantly, eliminating the pest infestation that caused it. A successful long-term strategy requires understanding the relationship between the pests and the mold.
Identifying the True Cause of the Black Fungus
Sooty mold is a fungus that grows on “honeydew,” not directly on the plant tissue. Honeydew is a sticky, sugary liquid excreted by sap-sucking insects as they feed on the tree’s phloem. This sweet residue allows the dark-colored fungus to flourish, coating leaves, branches, and trunks in a black film.
The primary culprits are aphids and the crepe myrtle bark scale (CMBS). Aphids typically feed on new, tender growth at the ends of branches. CMBS appear as white or gray felt-like encrustations, congregating on the bark, twigs, and in branch crotches. Crushing a CMBS insect will exude a distinct pink fluid, confirming its presence. Identifying the specific pest is important, as the two require different control methods.
Immediate Physical Removal of the Sooty Mold
Removing the visible sooty mold is an aesthetic treatment that also improves the tree’s ability to photosynthesize, but it does not treat the root cause. The mold can be gently washed from the leaves and bark using a solution of mild dish soap and water (about four ounces per gallon).
Apply the mixture to the affected areas, allow it to sit for a few minutes to soften the mold, and then thoroughly rinse it off with a strong stream of water. Ensure all soap residue is completely washed away to prevent potential leaf burn. For heavily coated trunks and branches, use a soft-bristle brush carefully to scrub the mold away. Pruning away small, heavily infected, and hard-to-reach branches is also a viable removal option.
Eliminating the Underlying Insect Infestation
Long-term control of sooty mold requires eliminating the sap-sucking insects that produce the honeydew. The treatment strategy depends on the specific pest and the severity of the infestation.
Aphid Control
Aphids are softer bodied and can often be managed with contact sprays such as insecticidal soaps or horticultural oils. These sprays work by smothering the pest upon direct contact and are applied during the active growing season. Horticultural oils, including neem oil, are effective against aphids and break down quickly, minimizing environmental impact.
Crepe Myrtle Bark Scale (CMBS) Control
Crepe myrtle bark scale (CMBS) is more difficult to control because it is protected by a waxy, felted covering, making a systemic approach necessary. A soil drench using systemic insecticides (like dinotefuran or imidacloprid) provides the most reliable control. These products are mixed with water and applied to the soil around the tree’s drip line in the early spring (typically late March to May) when the tree begins to leaf out. The tree absorbs the insecticide through its roots, distributing it throughout the plant tissue, which kills the scale insects as they feed.
Systemic treatments can provide control for an entire growing season, or even up to two years, but they must be applied before the tree blooms to protect pollinators. As an alternative, dormant oil applications can be made in late fall or early spring before new growth appears, smothering overwintering scale populations. Always follow the product label directions precisely when using any chemical treatment to ensure safety and effectiveness.
Cultural Practices for Long-Term Protection
Preventing the recurrence of sooty mold involves adopting maintenance practices that discourage pest populations and promote overall tree health. Proper pruning is a highly effective method, as removing overcrowded branches increases air circulation and light penetration throughout the canopy. Improved airflow creates an environment less favorable for both the pests and the subsequent mold growth.
Avoiding drought stress is also important, as stressed plants are more susceptible to insect infestations. Providing adequate water, especially during dry periods, helps the tree maintain its natural defenses. Excessive fertilization, which encourages rapid, tender new growth, should be avoided, as this soft tissue is a preferred feeding site for aphids.
Regular monitoring of the crepe myrtle, particularly during the spring and summer growing seasons, allows for the early detection of pests. Inspecting the undersides of leaves and the branch crotches for signs of stickiness or white scale is a simple way to catch a low-level infestation before it results in significant sooty mold accumulation. Addressing small pest problems immediately with horticultural oil or insecticidal soap prevents the need for more intensive chemical treatments later in the season.