The black coating on your crepe myrtle is not a disease attacking the tree itself. It is sooty mold, a superficial fungus that grows on the sticky honeydew left behind by sap-feeding insects. Treating it effectively means going after the insects first and the mold second, because scrubbing the mold off without dealing with the pest is like mopping a floor while the faucet is still running. The two most common culprits are crape myrtle bark scale and the crapemyrtle aphid, and each calls for a somewhat different approach.
Why Sooty Mold Shows Up in the First Place
Sooty mold belongs to a group of fungi, commonly species in the genus Capnodium, that feed on sugars rather than plant tissue. They do not penetrate bark or leaves. Instead, they colonize the honeydew that certain insects excrete after feeding on plant sap. Honeydew is essentially a concentrated sugar solution, and in warm, humid conditions it becomes a perfect growth medium. The mold spreads in a thin, dark layer that can coat stems and, in bad infestations, cover leaves as well.1HortScience. Crepe Myrtle Bark Scale and Sooty Mold Decrease Bloom and Fruit Number in ‘Natchez’ Crepe Myrtle Because it blocks sunlight from reaching leaf surfaces, heavy sooty mold reduces photosynthesis and can weaken the tree over time. But the mold is always a symptom, never the root cause.
Identifying the Insect Behind Your Sooty Mold
Before you reach for any product, figure out which pest is driving the honeydew. The two main offenders look and behave quite differently, and misidentifying the insect can lead you to the wrong treatment.
Crape Myrtle Bark Scale
Crape myrtle bark scale (CMBS) is a relatively recent arrival in the United States, first confirmed in Texas in 2004 and now spreading across the Southeast. The adult females look like small, white or grayish felt-like bumps along the bark, particularly in the crevices and branch crotches. If you crush one, it oozes pink or reddish fluid. Heavy infestations make the bark appear almost whitewashed, and the honeydew dripping from these colonies is what fuels the sooty mold beneath them. CMBS tends to produce especially heavy mold because the insects cluster on branches and trunks, so honeydew accumulates on every surface below.1HortScience. Crepe Myrtle Bark Scale and Sooty Mold Decrease Bloom and Fruit Number in ‘Natchez’ Crepe Myrtle
Crapemyrtle Aphid
The crapemyrtle aphid is a tiny, pale-green insect found mostly on the undersides of leaves. Unlike bark scale, these aphids are mobile and tend to appear in spring and summer as new growth flushes out. They reproduce rapidly and can coat leaf undersides in large numbers within a few weeks. The honeydew from aphids lands on leaves below the feeding site, so you often see sooty mold concentrated on the upper surfaces of lower leaves rather than on bark. If you flip a blackened leaf over and find clusters of small green insects, aphids are your problem.
Treating Crape Myrtle Bark Scale
CMBS is the harder of the two pests to control because the insects hide beneath a waxy covering on the bark, which shields them from contact sprays. Systemic insecticides applied as a soil drench tend to be the most effective option. Products containing imidacloprid or dinotefuran are the ones most commonly used. A soil drench delivers the active ingredient through the roots and into the sap, so when the scale insects feed, they ingest the insecticide from the inside out. Dinotefuran moves through the tree faster and is often recommended for quick knockdown, while imidacloprid is slower-acting but longer-lasting.
Research on ‘Natchez’ crepe myrtles showed substantial results from insecticide treatment over two growing seasons. In the first year, treated trees had sooty mold severity scores roughly 40% lower than untreated trees, and bloomed more than twice as much. By the second year, with continued treatment, sooty mold was nearly eliminated on treated trees while untreated trees still carried substantial mold, and treated trees produced more than double the blooms of untreated ones.1HortScience. Crepe Myrtle Bark Scale and Sooty Mold Decrease Bloom and Fruit Number in ‘Natchez’ Crepe Myrtle Fruit set told a similar story: treated trees bore roughly four times as many seed capsules as infested trees in the first year and about three and a half times as many in the second. The takeaway is that getting CMBS under control does not just improve appearance; it lets the tree bloom and reproduce at something closer to its normal capacity.
Timing matters. Soil drenches work best when applied in spring as the tree begins actively taking up water, roughly when new leaves are emerging. A second application in late summer can help control the next generation of crawlers. If you are using imidacloprid, keep in mind that it is toxic to pollinators. Applying it as a soil drench rather than a foliar spray reduces pollinator exposure, but if your crepe myrtle is in full bloom and attracting bees, you may want to time the drench for after the bloom period or switch to dinotefuran, which has a shorter residual in nectar.
Treating Crapemyrtle Aphids
Aphids are far easier to manage than bark scale. They have soft bodies and no protective covering, which means contact treatments work well. Insecticidal soap or horticultural oil sprayed directly onto aphid colonies will kill them on contact. You need good coverage of the leaf undersides, which is where the aphids congregate. A strong blast of water from a garden hose can also knock large numbers off the leaves. Aphids are poor fliers and many will not make it back to the tree.
Because aphids reproduce fast, a single treatment rarely solves the problem for the whole season. Plan on checking every week or two during the growing season and retreating if populations rebuild. Horticultural oil is a good choice because it also helps loosen existing sooty mold on the leaf surface, giving you a two-for-one effect.
In many cases, aphid populations stay manageable on their own thanks to natural predators like ladybugs, lacewings, and parasitic wasps. Heavy use of broad-spectrum insecticides for other garden problems can wipe out these beneficial insects and inadvertently cause aphid outbreaks. If your crepe myrtle aphid problem seems to come back every year despite treatment, consider whether something else in your spray program is killing the predators that would normally keep aphids in check.
Physically Removing the Sooty Mold
Once the insect population is under control and honeydew production drops, sooty mold will gradually weather away on its own. Rain and wind break it down over weeks to months. If you want to speed things up for cosmetic reasons, there are a few approaches that work.
For bark and branches, a pressure washer on a low setting or a garden hose with a spray nozzle can blast off most of the mold. Be careful with pressure washers on young or thin-barked branches, as too much force can damage the bark. For leaf surfaces, a dilute solution of horticultural oil or even a mild dish soap and water mixture sprayed on the foliage helps dissolve the sticky honeydew layer, and the mold flakes off with it. Some gardeners use a solution of about one tablespoon of baking soda per gallon of water, though this is more traditional advice than rigorously tested science.
There is no need to treat the mold with a fungicide. Sooty mold is not parasitic, and once the food source (honeydew) is gone, the fungus stops growing and gradually breaks apart. Spending money on fungicide sprays for sooty mold is treating a symptom that will resolve on its own once you deal with the insects.
Encouraging Natural Enemies
Biological control is worth thinking about, especially for CMBS, because chemical treatments need repeating and carry ecological trade-offs. Lady beetles are generalist predators that readily feed on crape myrtle bark scale.2PubMed. Evaluation of olfactory and visual cues for conservation biological control of crape myrtle bark scale in urban landscapes Research has explored ways to attract more of them to infested trees. In one study, yellow rectangular panels placed in the canopy of infested crepe myrtles led to a meaningful reduction in bark scale numbers on those trees.2PubMed. Evaluation of olfactory and visual cues for conservation biological control of crape myrtle bark scale in urban landscapes The panels appeared to recruit up to twice as many lady beetles as unaugmented trees, though the recruitment effect alone was not statistically significant; the scale reduction was. The idea is still being refined, but it suggests that even simple habitat modifications can tip the balance toward natural predators.
If you want to support beneficial insects around your crepe myrtles, avoid spraying broad-spectrum insecticides more than necessary. Planting companion plants that attract predatory insects, like yarrow, dill, or fennel, nearby can also help build a resident population of natural enemies. The goal is to create conditions where the pest population stays low enough that honeydew production never reaches the level that triggers visible sooty mold.
How Cultivar Choice Affects Your Risk
Not all crepe myrtle varieties attract the same pest loads, and choosing the right cultivar when planting a new tree can spare you significant trouble down the line. A study of 37 crepe myrtle cultivars in north Florida found wide variation in susceptibility to crapemyrtle aphids. The most resistant cultivar averaged about six aphids per leaf per sample date, while the most susceptible one averaged roughly 85.3Journal of Entomological Science. Susceptibility of Crape Myrtle, Lagerstroemia indica L., to the Crapemyrtle Aphid in North Florida That is more than a tenfold difference in pest pressure, which translates directly into more or less honeydew and more or less sooty mold.
Interestingly, the study found that cultivars with parentage from the Japanese species Lagerstroemia fauriei tended to carry higher aphid numbers than pure L. indica cultivars.3Journal of Entomological Science. Susceptibility of Crape Myrtle, Lagerstroemia indica L., to the Crapemyrtle Aphid in North Florida This is a genuine trade-off, because many popular hybrids with L. fauriei parentage were specifically bred for powdery mildew resistance and attractive bark. Varieties like ‘Natchez,’ ‘Muskogee,’ and ‘Tuscarora’ are favorites in the landscape for those traits, but they may come with greater aphid susceptibility. If sooty mold is a recurring problem in your area, it is worth weighting aphid resistance alongside mildew resistance when choosing a new tree.
Cultivars described as “tolerant” of powdery mildew, as opposed to those classified as either resistant or susceptible, tended to have lower aphid loads in the same study. This suggests that tolerance and outright resistance involve different plant characteristics, and the tolerant group may offer a middle ground for gardeners dealing with multiple pest issues at once.
The Real Cost of Leaving an Infestation Untreated
It is tempting to view sooty mold as just a cosmetic problem and leave it alone. The black coating is ugly, but if the tree seems otherwise healthy, many homeowners decide to wait it out. The research suggests this is a more consequential decision than it might seem. As noted earlier, untreated crepe myrtles with heavy bark scale infestations produced less than half the blooms and roughly a quarter of the fruit of treated trees.1HortScience. Crepe Myrtle Bark Scale and Sooty Mold Decrease Bloom and Fruit Number in ‘Natchez’ Crepe Myrtle For a tree planted specifically for its summer flowers, losing that much bloom is a significant hit to the reason you have the tree in the first place.
Beyond bloom loss, persistent infestations may slowly weaken the tree by reducing photosynthesis (from mold-covered leaves) and by diverting sap to feed insect colonies. A tree under chronic stress is more vulnerable to secondary problems like opportunistic diseases, drought damage, and winter injury. The decline is gradual enough that it can be hard to attribute to any single cause, but season after season of heavy pest pressure adds up.
How Bark Scale Changes the Insect Community Around Your Tree
One underappreciated consequence of a CMBS infestation is that it reshapes the entire insect community living on and around the tree. Research comparing infested and non-infested crepe myrtles found that infested trees supported dramatically higher insect abundance overall. In midsummer, infested trees had about twice the total insect abundance of clean trees, and even earlier in the season the difference was more than 50%.4PubMed Central. Crape myrtle bark scale Acanthococcus lagerstroemiae infestation seasonally alters the insect biodiversity on crape myrtle trees The infestation status of the tree explained a substantial share of the variation in community composition across sites.
This is not necessarily all bad. Some of those additional insects are predators and parasitoids that feed on the scale, which is the beginning of a natural biological control response. But the influx also includes ants, which farm scale insects for their honeydew and actively protect them from predators, and various scavengers attracted to the sugar-rich environment. If you have noticed more ant activity on a sooty crepe myrtle, this is why. Managing ants with bait stations near the trunk can help tip the predator-prey balance back in favor of the beneficial insects that eat scale. Sticky barriers or trunk wraps that block ant trails up the bark are another low-tech option that pairs well with any predator-based control strategy.
A Seasonal Game Plan
Putting all of this together, here is a practical timeline for managing sooty mold through the year:
- Late winter: Inspect bark closely for white, felt-like scale colonies. Scrub accessible ones off with a soft brush and soapy water. This is also a good time to apply dormant horticultural oil, which smothers overwintering scale and eggs.
- Early spring: Apply a systemic soil drench (dinotefuran for fast action, imidacloprid for longer residual) as new growth begins, if CMBS was present the previous year. Avoid foliar application during bloom to protect pollinators.
- Late spring through summer: Monitor leaf undersides for aphids. Treat with insecticidal soap or horticultural oil as needed. Hose down foliage after treatment to wash away honeydew and loose mold.
- Midsummer: Check bark for a second generation of CMBS crawlers. A second systemic treatment or a targeted contact spray on crawlers may be warranted.
- Fall: After leaves drop, pressure-wash heavy mold from trunks and main branches. Remove fallen leaves that may harbor scale or mold spores.
Gardeners in areas where CMBS has not yet appeared should keep an eye on regional extension service alerts. The pest is still spreading, and early detection makes control far easier than trying to beat back an established infestation. If you spot unfamiliar white bumps on crepe myrtle bark, especially if the tree is developing sooty mold when it never did before, report it to your local cooperative extension office so they can track the spread and advise you on the best response for your area.