The dark, powdery coating often described as “black mold” on crepe myrtle trees is scientifically known as sooty mold, a common fungal issue. This superficial growth typically appears as a thin, black film that covers the leaves, stems, and sometimes the trunk of the plant. Unlike true plant diseases that invade and destroy tissue, sooty mold fungi do not directly infect the crepe myrtle itself. Instead, they form a dense, unsightly layer over the plant’s surface, which can sometimes reduce photosynthesis by blocking sunlight.
Why Black Mold Appears
Sooty mold growth is a secondary symptom triggered by the presence of sap-sucking insects on the crepe myrtle. These pests consume large amounts of the plant’s nutrient-rich sap, excreting the excess sugars as a sticky substance called honeydew. Honeydew adheres to the plant surfaces below the feeding site, providing a carbohydrate-rich substrate upon which various species of mold fungi, such as those in the genus Capnodium, can thrive.
The primary culprits responsible for this sticky residue are often Aphids, especially during the spring, or the more persistent Crapemyrtle Bark Scale (CMBS). CMBS appears as small, pinkish-gray felted insects clustered in branch crotches and on the trunk. It is particularly problematic because it produces copious amounts of honeydew throughout the growing season.
Immediate Physical Removal of the Mold
The immediate action to improve the tree’s appearance involves physically removing the layer of sooty mold and the underlying honeydew. For light infestations, a strong stream of water from a garden hose can often dislodge the film from the leaves and branches. Applying water pressure carefully ensures the mold is rinsed away without damaging the foliage.
For more persistent or heavy coatings, a mild cleaning solution may be necessary to break down the sticky honeydew. A mixture of mild dish soap, approximately two to three tablespoons per gallon of water, can be sprayed onto the affected areas. Allowing this solution to sit for a few minutes before rinsing thoroughly with clean water helps to loosen the mold’s grip on the plant surface.
Horticultural soaps designed specifically for plants are also effective and are generally considered safer for the crepe myrtle’s delicate tissues. When applying any solution, thorough coverage is important, particularly on the underside of leaves where the honeydew may accumulate. Heavily coated or dense interior branches that are difficult to clean can be strategically pruned away.
Removing these sections improves the tree’s overall appearance and increases air circulation within the canopy, which helps to reduce future fungal growth. This cleaning step should be repeated as needed throughout the growing season while long-term pest control methods take effect. The dense mold coating can impede the plant’s ability to conduct photosynthesis effectively.
Eliminating the Pest Infestation
The long-term solution requires eliminating the sap-sucking insects responsible for honeydew production.
Systemic Control
For Crapemyrtle Bark Scale (CMBS), which is a particularly difficult pest to manage, systemic insecticides often offer the most reliable control method. These products, which contain active ingredients like dinotefuran or imidacloprid, are applied to the soil around the base of the tree in early spring. The tree’s root system absorbs the chemical, distributing it throughout the plant’s vascular system, thereby poisoning the scale insects as they feed on the sap. Proper timing is important because the insecticide needs time to move through the plant before the scale populations reach their peak in mid-summer. Applying these products too late significantly reduces their efficacy against CMBS.
Contact Treatments
Horticultural oils, specifically the lighter summer-weight oils, provide a contact method of control. These oils work by suffocating the insects, requiring complete coverage of the scale colonies, particularly those nestled in the crevices of the bark and branch crotches. Application of these oils should be done when temperatures are moderate, typically below 85 degrees Fahrenheit, to avoid phytotoxicity (damage to the plant tissue caused by the oil).
Insecticidal soaps provide another contact option, which disrupts the cell membranes of soft-bodied insects like aphids and the crawler stage of CMBS. Multiple applications are often necessary with soaps and oils, as they only kill the pests present at the time of spraying and leave no residual protection. For aphids, targeted applications of insecticidal soap or a strong jet of water can often keep populations in check.
Monitoring the tree throughout the year for the reappearance of honeydew or the insects is an important part of the management strategy. Combining systemic treatments for persistent scale with topical sprays for immediate aphid control provides a comprehensive approach to removing the source of the sooty mold problem. Consistent management of the underlying insect population prevents the recurrence of the black sooty mold.