Sooty mold on crepe myrtles is a black, soot-like coating caused by fungi feeding on honeydew, a sticky substance left behind by sap-sucking insects. The mold itself doesn’t infect the plant directly. It grows on the surface of leaves, branches, and bark, blocking sunlight and making the tree look like it’s been dusted with charcoal. To get rid of it, you have to get rid of the insects producing the honeydew.
Why Sooty Mold Appears
Sooty mold fungi (primarily Capnodium species) don’t attack crepe myrtle tissue. They colonize honeydew, a sugary liquid excreted by insects that pierce leaves and bark to drink plant sap. The insects are the root cause. Without them, there’s no honeydew, and without honeydew, the mold has nothing to grow on.
The most common culprit is the crepe myrtle aphid, a small yellowish-green insect that clusters on the undersides of new leaves. Aphids insert needlelike mouthparts into tender growth and excrete large amounts of honeydew as they feed. Other insects that trigger the same cycle include scales, mealybugs, and whiteflies. In recent years, crepe myrtle bark scale has become a major contributor across the southeastern United States and is now the leading cause of heavy sooty mold buildup on trunks and branches.
How to Identify the Insect Behind It
Figuring out which insect is driving the problem determines how you treat it. Here’s what to look for:
- Crepe myrtle aphids: Tiny, pale green insects found on the undersides of leaves, especially new growth. They’re most active in spring and summer. Leaves may curl or yellow, and you’ll notice a shiny, sticky residue on foliage and anything parked beneath the tree.
- Crepe myrtle bark scale: Look for small white, felt-like bumps (about 1/8 inch long) clustered along the bark, especially near branch crotches. If you crush one, it bleeds a pinkish-red color. This is the telltale sign. Bark scale produces enough honeydew to coat entire trunks in thick black mold, which is why it’s often the first thing homeowners notice.
Bark scale has two generations per year. Eggs laid in late April hatch about three weeks later in early May. Those nymphs mature over the summer and produce a second generation that hatches in September. The newly hatched nymphs, called crawlers, are tiny and pink. They settle into bark crevices, insert threadlike mouthparts, and begin feeding and producing honeydew almost immediately.
What Sooty Mold Does to the Tree
Sooty mold won’t kill a crepe myrtle, but heavy coverage weakens it over time. The black film coats leaf surfaces and blocks sunlight, reducing the tree’s ability to photosynthesize. A tree with thick mold coverage produces less energy, which can mean fewer blooms, slower growth, and a generally stressed appearance. The insects themselves also drain resources by continuously siphoning sap. Together, the feeding damage and reduced sunlight put the tree in a cycle of declining vigor until the insect population is controlled.
How to Clean Off the Mold
You can wash sooty mold off bark and trunks with warm, soapy water and a soft brush. This improves the tree’s appearance immediately and lets more light reach the bark surface. On leaves, rain and natural weathering will break down the mold once the honeydew supply stops. The key point: cleaning removes the symptom, not the cause. If the insects are still feeding, the mold will return within weeks.
Treating the Insects
For Aphids
Crepe myrtle aphids are often managed by natural predators, including ladybugs, lacewings, and parasitic wasps. A strong blast of water from a garden hose knocks aphids off leaves and can reduce populations enough to limit honeydew production. For heavier infestations, insecticidal soap or horticultural oil sprayed directly onto affected foliage kills aphids on contact without leaving long-lasting residues.
For Bark Scale
Bark scale is harder to control because the insects are protected under waxy coverings for most of their life cycle. The most effective treatment is a soil-applied systemic insecticide, which the tree absorbs through its roots and distributes through its sap. When scale insects feed, they ingest the treatment.
To apply a soil drench, rake mulch away from the base of the tree, mix the product in a watering can according to label directions, and pour it evenly around the root zone. The optimal window is late March through May, targeting the first generation of nymphs for the year. You can apply from the time trees leaf out in spring until leaves begin changing color in fall, but earlier applications give better results. Apply only one soil drench per year.
For the crawler stages specifically, foliar sprays containing insect growth regulators can be effective. Late spring and late summer, when crawlers are active and exposed, are the two best windows for spray applications.
Pollinator Considerations
The systemic products used for bark scale belong to the neonicotinoid class of insecticides, which are toxic to bees and other pollinators. Crepe myrtles are heavy summer bloomers that attract pollinators in large numbers. Applying soil drenches in the recommended late March through May window, well before peak bloom in most regions, helps reduce the concentration present in flowers by the time pollinators visit. Even so, if your trees are heavily visited by bees, weigh the severity of the infestation against the risk. For light infestations, washing the trunk and encouraging natural predators may be sufficient without systemic treatments.
Preventing Future Buildup
Good air circulation reduces the conditions that favor both insects and fungal growth. Avoid planting crepe myrtles in crowded spots, and resist the urge to “crepe murder” (topping the tree aggressively), which produces dense clusters of weak new growth that aphids love. Proper pruning that opens the canopy lets air flow through and makes the tree less hospitable to pests.
Inspect your crepe myrtles regularly from spring through fall. Catching aphids or bark scale early, before honeydew accumulates, prevents the sooty mold cycle from getting established. A few white bumps on the bark or a handful of aphids on new leaves are far easier to deal with than a tree already coated in black mold. If you’re planting new crepe myrtles, some cultivars show better resistance to aphids, so checking with your local extension office for recommended varieties can save you trouble down the road.