Sooty mold is a black coating that develops on the leaves, branches, and trunks of plants, including the crepe myrtle. This dark residue is aesthetically unappealing. While the mold is not a direct threat to the plant’s tissue, a severe coating can interfere with photosynthesis by blocking sunlight. Sooty mold is a common issue for crepe myrtles, indicating an underlying pest problem that requires targeted management.
Understanding the True Cause of Sooty Mold
The black layer is a fungus that grows only on honeydew, a sugary substance. Honeydew is an excretion left behind by sap-feeding, piercing-sucking insects. These insects consume the plant’s vascular fluid and excrete the unused, sugar-rich liquid onto plant surfaces. Controlling the mold depends entirely on eliminating the insects that produce this food source.
Two pests are responsible for sooty mold on crepe myrtles. The Crepe Myrtle Aphid is a soft-bodied insect that feeds on new growth and leaves, causing honeydew to drip onto lower foliage. The Crapemyrtle Bark Scale (CMBS) appears as small, white or gray felt-like encrustations, often in branch crotches and on the bark. When these scale insects feed on trunks and branches, they excrete honeydew directly onto the bark, leading to mold buildup.
Essential Strategies for Pest Management
Controlling these pests involves cultural care, organic treatments, and systemic options. Proper plant maintenance is the first line of defense, as healthy trees are less susceptible to infestations. Avoid excessive nitrogen fertilizer, which encourages the new growth that aphids favor. Good air circulation through selective pruning also reduces hiding spots for pests and limits humid conditions that favor mold growth.
A strong blast of water from a hose can dislodge soft-bodied pests like aphids from the leaves. Horticultural oils (dormant or summer oils) and insecticidal soaps offer effective organic treatment for aphids and the vulnerable “crawler” stage of scale insects. These products smother the pests, requiring thorough coverage of all infested surfaces, including leaf undersides and bark crevices. Always check the label before application to ensure the product is safe for crepe myrtles and the current temperature range.
Infestations involving Crapemyrtle Bark Scale may require a systemic insecticide. These products are absorbed by the plant and move through the tissues, killing sap-feeding pests. A soil drench containing imidacloprid is often applied to the root zone early in the season, usually in late April or early May. This timing allows the chemical sufficient time to move throughout the tree before summer pest populations peak.
Systemic Trunk Application
Another systemic option is painting a paste containing an insecticide like acephate directly onto the trunk. The tree’s thin, exfoliating bark allows the product to be absorbed, providing protection as the chemical moves through the plant’s system. Systemic treatments must be used carefully according to label instructions to minimize impact on pollinators during the blooming period. Focusing on control early in the growing season, before pest numbers explode, is the most effective prevention against sooty mold development.
Cleaning Existing Sooty Mold Residue
Once the insect infestation is managed and the flow of honeydew stops, the sooty mold will dry and flake away naturally. This process can take time and may not clear the black residue from heavily coated areas. To speed up the tree’s restoration, you can manually clean the affected leaves and bark.
A simple cleaning solution involves mixing mild dish soap with water. Gently washing the affected surfaces with this solution, followed by a rinse with clean water, helps dissolve the honeydew and lift the mold. It is important to wait until the pest population is under control before cleaning. Removing the mold without eliminating the honeydew source will result in the black coating returning within a few weeks. Focus on the most visible areas, as successful pest management is the solution to long-term prevention.