How to Get Rid of Black Fungus on Crepe Myrtles

The black coating on crepe myrtles is almost never an infection of the tree itself. It is sooty mold, a superficial fungus that grows on the sticky honeydew left behind by sap-sucking insects, most commonly crepe myrtle bark scale. Removing it means tackling the insects first and then washing the mold off, because the fungus will return as long as the pests keep feeding. The good news is that sooty mold does not penetrate bark or leaf tissue the way a true plant disease would, so once you break the cycle of insect infestation, the tree can recover fully.

Why It Looks Like the Tree Is Diseased

Sooty mold is a group of fungi, often in the genus Capnodium, that colonize any surface coated in sugary honeydew. The mold is dark gray to jet black, sometimes velvety, and it clings to branches, trunks, and leaves. Because it can cover large swaths of bark, people often assume the tree has some kind of fungal rot or blight. In reality, the mold is feeding on insect excrement, not on the tree. It sits on the surface like a coat of paint rather than growing into the wood.

That does not mean sooty mold is harmless. When the layer becomes thick enough to coat leaves, it blocks sunlight and slows photosynthesis. A two-year study on ‘Natchez’ crepe myrtles found that untreated trees with heavy bark scale and sooty mold produced roughly half as many blooms as treated trees in the first year and less than half in the second year.¹ Fruit production dropped by a comparable margin. So while the mold will not kill the tree, it absolutely steals vigor and flowers.

Identify the Insect Producing the Honeydew

You cannot solve sooty mold without figuring out which insect is generating the honeydew. On crepe myrtles, the most common culprits are:

Crepe myrtle bark scale (CMBS): A relatively recent arrival in the United States, now widespread across the Southeast and spreading. The insects look like small white or gray felt-like bumps on the bark, often concentrated in branch crotches and on the undersides of limbs. Female CMBS produce copious honeydew. When you crush one, it oozes pink blood-like fluid, which is a reliable field ID test.
Aphids: Crepe myrtle aphids are tiny, yellowish-green insects that cluster on new growth and the undersides of leaves in spring and early summer. They also excrete honeydew, though usually less of it than bark scale.
Soft scale and whiteflies: Less common on crepe myrtles than the first two, but both produce honeydew and can fuel sooty mold in the right conditions.

CMBS deserves special attention because it has become the dominant driver of severe sooty mold on crepe myrtles across Texas and the broader Gulf states in recent years, and infestations can be dramatically worse than what aphids alone produce. If your crepe myrtle’s trunk and main scaffold branches are encrusted in black, bark scale is the most likely explanation.

Step One: Physically Wash It Off

Before reaching for any chemical, start with a pressure washer or a strong garden-hose nozzle. In the study mentioned above, researchers removed all visible signs of bark scale and sooty mold from every tree using a high-pressure water spray at the start of the second growing season.¹ This did not eradicate the scale permanently, but it gave the treated trees a clean starting point. By season’s end, treated trees had sooty mold scores near zero, while untreated trees still carried visible mold.

Pressure washing works because sooty mold is superficial. You are literally blasting off a surface coating along with any exposed scale insects and egg masses. A few practical tips: do this in late winter or very early spring before new growth emerges, use moderate pressure so you do not strip live bark, and focus on the trunk and main branches where the mold and scale are densest. On leaves, a strong spray of water from a hose is usually enough; the mold peels away easily once it is wet.

Some gardeners add a dilute solution of horticultural oil or dish soap to the water to help loosen the mold and smother any remaining crawlers. This is a reasonable approach, but the mechanical force of the water does most of the work.

Controlling the Insects That Feed the Mold

Physical removal is a reset button, not a cure. If the insects remain, honeydew production resumes, and the mold comes back. Controlling the pest population is the step that actually breaks the cycle.

Systemic Insecticides

For bark scale infestations, the most effective approach documented in research is a soil drench with a systemic neonicotinoid such as imidacloprid or dinotefuran. The insecticide is taken up through the roots and distributed through the tree’s vascular system, reaching the sap the scale insects feed on. In the two-year ‘Natchez’ trial, trees treated with a systemic insecticide had dramatically lower bark-scale and sooty-mold scores by the end of both seasons and more than doubled their bloom counts compared to untreated trees.¹ By the second year, treated trees were producing over 5,000 blooms versus about 2,100 for untreated ones, and fruit numbers followed a similar pattern.

Dinotefuran tends to move through the tree faster than imidacloprid, which matters if you need quicker knockdown of a heavy infestation. Imidacloprid is slower to distribute but persists longer. Many extension services recommend applying the drench in spring when the tree is actively growing and transpiring, because that is when uptake is fastest. A fall application can also work, especially in warmer climates where crepe myrtles retain some metabolic activity late into the year.

Contact and Horticultural Oil Sprays

If the infestation is moderate or you prefer not to use systemics, dormant-season horticultural oil sprays can smother overwintering scale crawlers and eggs. Apply during late winter when the tree is leafless and crawlers are dormant but exposed. During the growing season, lightweight summer oils can be sprayed on foliage and younger bark. These oils work on contact rather than systemically, so thorough coverage is critical. For aphids, a strong jet of water knocks most of them off, and insecticidal soap finishes what the water does not reach.

Pollinator Safety and Neonicotinoid Concerns

Any discussion of systemic insecticides for crepe myrtles needs to address pollinators, because crepe myrtles bloom heavily and attract bees. Neonicotinoids like imidacloprid are taken up into all parts of the plant, including the nectar and pollen of flowers. Research on ornamental plants treated with imidacloprid has found the insecticide present in floral nectar at concentrations well above background levels. In one study on ornamental phacelia, nectar from plants given a high dose of imidacloprid contained roughly 55 parts per billion, while even the low-dose treatment produced detectable residues.² Solitary bees exposed to treated flowers in that study showed reduced nesting success.

Work on cucurbit crops paints a consistent picture: systemic insecticides including imidacloprid and thiamethoxam regularly exceeded concern thresholds in flower structures after soil applications.³ While these studies were not conducted on crepe myrtles specifically, the underlying mechanism is the same: a soil-applied systemic insecticide moves into the flowers.

If you want to minimize pollinator exposure while still using a systemic drench, timing is your best tool. Apply the drench well before bloom season so that concentrations in nectar are lower by the time flowers open. In practice, that means a late-fall or very early spring application rather than a summer one. Alternatively, dinotefuran breaks down faster than imidacloprid, which may reduce the window of floral contamination, though direct data on crepe myrtle nectar residues is limited. Horticultural oils and insecticidal soaps, by contrast, pose virtually no risk to pollinators once the spray has dried.

Cultural Practices That Reduce Infestations

Chemical control is the fastest fix for a heavy bark-scale problem, but a few management habits can make infestations less likely or less severe over time.

Pruning for good air circulation matters. Dense, congested canopies hold humidity and create sheltered spots where scale insects thrive. Removing inward-growing branches, water sprouts, and crossing limbs opens the canopy to air and light, which discourages both the insects and the mold. This is also the time to remove any heavily encrusted branches that are beyond saving, cutting them back to clean wood.

Soil nutrition is another lever, and it works in a counterintuitive direction. Laboratory research has shown that crepe myrtle bark scale actually performs better on nutrient-deficient host plants. Scale insects reared on plants grown under nutrient-poor conditions had higher reproduction and faster population growth than those on well-nourished plants.⁴ Keeping your crepe myrtle in good overall health with appropriate fertilization does not just make the tree look better; it appears to make the tree a less favorable host for the scale.

Avoid over-fertilizing with nitrogen-heavy formulas, though. Excessive nitrogen produces lush, soft new growth that aphids adore. A balanced slow-release fertilizer applied once in spring is generally enough for an established crepe myrtle.

Encouraging Natural Enemies

Lady beetles, lacewings, and parasitic wasps all feed on or parasitize crepe myrtle bark scale and aphids. In landscapes where broad-spectrum insecticides are not routinely sprayed, these beneficial insects can keep scale populations from exploding. Twice-stabbed lady beetles in particular have been observed feeding voraciously on CMBS in the field. If your infestation is moderate and you can tolerate the cosmetic damage of sooty mold for a season, letting natural enemies build up their populations before intervening with chemicals can result in longer-term balance. Heavy systemic insecticide use, by contrast, tends to suppress the beneficials along with the pests, which sets the stage for rapid rebound once the chemical wears off.

One practical middle ground is to apply a soil drench of systemic insecticide in the first year to knock the infestation down to manageable levels, then rely on physical removal and natural predators in subsequent years. The two-year research trial mentioned earlier saw dramatic improvement in both scale and sooty mold from the first to the second year, suggesting that knocking down the population once can shift the balance enough for ongoing management to be lighter-touch.

Choosing a Less Susceptible Variety

Not all crepe myrtle species and cultivars are equally attractive to bark scale. Research on the biological development of CMBS across different host plants has found that the insect’s performance, including survival and reproduction, varies significantly depending on the crepe myrtle variety it feeds on.⁵ Some hosts slow down scale development and reduce fecundity, while others allow the insects to thrive.

If you are planting new crepe myrtles or replacing one that has been badly damaged, it is worth asking your local extension office which varieties have shown more tolerance or resistance to CMBS in your region. The research on host-plant resistance is still evolving, and formal resistance ratings for named cultivars are not yet standardized. But the underlying biology is clear: the variety you choose changes how fast scale populations build, which in turn changes how much sooty mold you end up dealing with.

When the Mold Keeps Coming Back

If you have washed the mold off, treated for insects, and it still returns within the same season, the most common explanations are incomplete insect control or re-infestation from nearby untreated trees. CMBS crawlers are mobile and can travel between trees, and adult females on a neighboring crepe myrtle can produce enough crawlers to recolonize your tree within weeks. In neighborhoods where crepe myrtles line every street, coordinated treatment among neighbors makes a real difference. One untreated tree upwind can serve as a reservoir for the entire block.

Another possibility is that the honeydew is coming from above rather than from the crepe myrtle itself. If a larger tree overhangs the crepe myrtle and hosts aphids or other honeydew-producing insects, the drip can land on the crepe myrtle’s canopy and fuel sooty mold growth. In that case, no amount of treatment on the crepe myrtle will solve the problem. Look up. If the overhanging tree is the source, addressing the pest on that tree, or pruning back the overhanging branches, is the real fix.

Finally, some gardeners mistake other dark growths for sooty mold. Lichens and algae can colonize crepe myrtle bark, especially in humid climates, and may appear dark green or blackish. These are generally harmless to the tree and do not indicate an insect problem. If the dark growth scrapes off as a powdery or flaky layer and there is sticky residue underneath, it is sooty mold. If it is tightly adhered to the bark, greenish up close, and there is no stickiness, it is probably lichen or algae, neither of which requires treatment.