Crepe Myrtle (Lagerstroemia indica) is a popular ornamental tree known for its extended summer bloom and beautiful, often multi-trunked structure. A common sight is the sudden appearance of a dark, crusty coating that makes the tree look dirty. This black discoloration, which can cover leaves, twigs, and the trunk, signals an underlying issue affecting the tree’s appearance and health. Understanding this black film is the first step toward restoring the tree’s vibrant aesthetics.
Sooty Mold: The Primary Cause of Blackening
The black substance coating your Crepe Myrtle is a fungal growth known as Sooty Mold. This fungus is not parasitic; it does not directly infect the plant tissue. Instead, Sooty Mold is a saprophyte, surviving exclusively on “honeydew,” the sugary waste product secreted by certain insects.
The fungal growth forms a dense, superficial layer that can look powdery, velvety, or crusty. While the mold is harmless to the plant’s internal structure, a heavy coating creates a physical barrier on the leaves. This black layer blocks sunlight, reducing photosynthesis. This reduction in energy production can lead to weakened growth and reduced flowering, making pest control the priority.
Identifying the Pest Infestation Driving Honeydew Production
The presence of Sooty Mold indicates your tree is infested with sap-sucking insects that excrete honeydew. The two most common culprits are Crepe Myrtle Aphids and Crepe Myrtle Bark Scale (CMBS). Aphids are small, pale yellowish-green insects that congregate on new growth and the undersides of leaves. They feed on tender tissue and rapidly reproduce, leading to widespread honeydew accumulation.
Crepe Myrtle Bark Scale is a felt-like scale insect that poses a greater long-term threat. It is often found as white or gray clusters colonizing the trunk, branches, and sheltered areas of branch crotches. Infested bark often appears encrusted. Since both pests secrete honeydew, controlling the insect population is the only way to stop the production of the Sooty Mold’s food source and resolve the blackening.
Immediate Cleaning and Treatment Protocol
The remediation process involves two actions: physically removing the black mold and treating the underlying insect infestation. To address the immediate aesthetic problem, wash the affected areas with a strong stream of water from a hose. For a more thorough cleaning, apply a solution of mild dish soap and water to the leaves and bark, let it sit, and then thoroughly rinse. This removes the sticky honeydew and mold, improving the tree’s appearance and light absorption.
Pest control requires a targeted approach based on the insect identified. For lighter aphid infestations, horticultural oils or insecticidal soaps are effective, as they must contact the soft-bodied insects directly. These products are less harmful to beneficial insects than broad-spectrum sprays.
Systemic Treatment for Severe Infestations
More severe infestations, especially those involving Crepe Myrtle Bark Scale, often require systemic insecticides, such as dinotefuran or imidacloprid. Systemic treatments are typically applied as a soil drench around the base of the tree in early spring. The insecticide is absorbed by the roots and translocated throughout the plant, providing season-long control.
It is recommended to wait until after the tree has finished blooming to apply systemic insecticides, as they can harm pollinators. Always adhere to product label instructions for effective and safe management.
Differentiating Normal Bark Exfoliation from Disease
Not all darkening on a Crepe Myrtle is a sign of disease or pest infestation. As the tree matures, it regularly sheds its thin, outer bark in strips or flakes, a process called exfoliation. This shedding reveals the smooth, inner bark, which is often colored in patches of tan, gray, cream, or cinnamon. This mottled appearance is a prized ornamental feature, especially during the winter.
This normal, healthy coloring is smooth to the touch and should not be confused with the rough, crusty, and uniformly black texture of Sooty Mold. If the discoloration is a smooth, colorful pattern, it is a healthy sign of a maturing tree requiring no intervention. If the bark is peeling and you see patches of white, felt-like scale insects underneath, this indicates a CMBS problem.