Black Sooty Mold? - Ask Extension
Hi Ask Extension colleagues (I answer questions on perennials and annuals).
I recently noticed some severe disease on my very close-by neighbor's...
Knowledgebase
Black Sooty Mold? #948155
Asked November 05, 2026, 4:35 PM EST
Hi Ask Extension colleagues (I answer questions on perennials and annuals).
I recently noticed some severe disease on my very close-by neighbor's crepe myrtle and adjacent spotted laurel (aucuba japonica). Aside from wanting my neighbor's plants to look good, I am concerned about disease spreading to my garden beds nearby.
The crepe mrytle has a black coating on several of the trunks with little white specks. I'm guessing black sooty mold caused by aphids, though I realize the mold can be caused by several types of pests. The portions of the aucubas on either side of the crepe myrtle are also severly infected with some pathogen, and a significant number of limbs have died altogether. See photos. The problem, at least so far, is localized, and I haven't seen sign of disease on any other crepe myrtles or other shrubs. The affected areas of the aucuba are under the branches of the crepe myrtle and largely shaded; the portions in sunny locatiions still look healthy.
I first noticed the black coating on the crepe myrtle several weeks ago, but only today did I observe the disease on the aucubas. While it's possible that I missed the disease on the aucubas when I looked previously, I think that unlikely. Accordingly, I think that what I'm seeing is the disease spreading. It seems that it started on the crepe myrtle and spread outwards to either side to the aucubas.
I welcome your help both with the diagnosis and recommended treatment. I have read about treatiing the crepe myrtle first with insecticidal soap and then with a strong rinse from a hose, but I don't have any idea what to do about treating the aucubas. What to do you recommend? Also, is my instinct correct that my neighbor should--promptly--remove and destroy all dead limbs? Should she also raise the canopy of the crepe myrtle to provide more air circulation for the aucubas underneath?
Thanks so much for your help. Please don't hesitate to reach out if you need more information. Also I can more pictures if that would be helpful.
Bob Loesche
Montgomery County Maryland
Expert Response
The black coating pictured is definitely sooty mold, and as such it's growing on honeydew produced by a sap-sucking insect. Sooty mold not a disease because it doesn't directly infect plants, merely growing on the honeydew residue instead. Indirectly, it can weaken plants if too much foliage is covered for a long period of time that reduces photosynthesis, but the same could be said for a coating of, say, spray-paint, so its presence is causing more of an impact on growing conditions than being a disease per se.
We see a few Crapemyrtle Bark Scale on the trunk of the crapemyrtle, although the population doesn't look large enough to be creating that much honeydew by themselves, unless they are more abundant on higher branches that we can't see. A combination of scale honeydew with that of Crapemyrtle Aphids or Spotted Lanternfly (SLF) is also possible. If you didn't notice sooty mold buildup until recently, perhaps SLF adults are the main cause since they produce more honeydew than nymphs.
The sooty mold will eventually weather off, especially once the fungus runs out of honeydew to feed on, both due to precipitation rinsing any buildup off (or diluting it) and due to the insects pausing or slowing their feeding for the winter. Some of it is already flaking away from the waxy leaf surfaces of the cherrylaurel (the evergreen shrubs below the windows in front, likely variety 'Otto Luyken') and probably on the aucuba as well (the evergreen shrub on the other side of the crapemyrtle).
Some formulations of horticultural oil can be used to speed-up the deterioration and detachment of sooty mold, but check the product label for usage instructions as not all hort oil products may be effective for that use. Hort oil can also be used to treat the scale (and will probably work better than insecticidal soap), either in dormancy as a "dormant oil" spray and/or in the growing season as a "summer rate" spray. Systemic insecticide can be used (applied by a licensed applicator) if no other control method works well enough for the scale and/or aphids, although it should be a last resort. It's not practical to use an insecticide if SLF are the main issue, however.
The crapemyrtle appears to be airy enough in the canopy that we don't think much improvement in air circulation would be had by pruning it. Good air circulation is helpful for avoiding leaf spot infections and similar diseases, but in this case, with sooty mold not being an infectious disease, it's less important because the fungus is not relying on wet surfaces or high humidity to infect a leaf.
We can't see aucuba dieback in the photos given the extent of the sooty mold, but if branches on that shrub died, it might be a consequence of drought stress, such as from Botryosphaeria canker infection. There is no curative remedy for any branch dieback other than to prune out the damage and to try to alleviate plant stress from environmental conditions like insufficient rain, which will make the plant less vulnerable to those kinds of opportunistic infections. Aucuba doesn't have many pest problems, and once symptoms manifest for a fungal disease, it would be too late to use a fungicide anyway since they are only preventative measures. (And not every fungicide ingredient works equally well for each potential fungal pathogen, so it would ideally need to be identified first based on early-stage symptoms.)
For now, see if you can determine if scale are abundant on the crapemyrtle's bark, or if SLF adults are present. It's late in the season for Crapemyrtle Aphids to be a problem, but you may be able to see evidence of their earlier presence if fallen leaves have shed skins stuck to them. I would not worry about any aphids this time of year if you do find some. Since many areas are still abnormally dry, the plants can be checked for watering needs to reduce any stress they are under from the drought that isn't over yet, unless the neighbor was irrigating them periodically this summer. A strong blast of hose water on the foliage and bark might also help remove some of the sooty mold and scale insects.
Miri
We see a few Crapemyrtle Bark Scale on the trunk of the crapemyrtle, although the population doesn't look large enough to be creating that much honeydew by themselves, unless they are more abundant on higher branches that we can't see. A combination of scale honeydew with that of Crapemyrtle Aphids or Spotted Lanternfly (SLF) is also possible. If you didn't notice sooty mold buildup until recently, perhaps SLF adults are the main cause since they produce more honeydew than nymphs.
The sooty mold will eventually weather off, especially once the fungus runs out of honeydew to feed on, both due to precipitation rinsing any buildup off (or diluting it) and due to the insects pausing or slowing their feeding for the winter. Some of it is already flaking away from the waxy leaf surfaces of the cherrylaurel (the evergreen shrubs below the windows in front, likely variety 'Otto Luyken') and probably on the aucuba as well (the evergreen shrub on the other side of the crapemyrtle).
Some formulations of horticultural oil can be used to speed-up the deterioration and detachment of sooty mold, but check the product label for usage instructions as not all hort oil products may be effective for that use. Hort oil can also be used to treat the scale (and will probably work better than insecticidal soap), either in dormancy as a "dormant oil" spray and/or in the growing season as a "summer rate" spray. Systemic insecticide can be used (applied by a licensed applicator) if no other control method works well enough for the scale and/or aphids, although it should be a last resort. It's not practical to use an insecticide if SLF are the main issue, however.
The crapemyrtle appears to be airy enough in the canopy that we don't think much improvement in air circulation would be had by pruning it. Good air circulation is helpful for avoiding leaf spot infections and similar diseases, but in this case, with sooty mold not being an infectious disease, it's less important because the fungus is not relying on wet surfaces or high humidity to infect a leaf.
We can't see aucuba dieback in the photos given the extent of the sooty mold, but if branches on that shrub died, it might be a consequence of drought stress, such as from Botryosphaeria canker infection. There is no curative remedy for any branch dieback other than to prune out the damage and to try to alleviate plant stress from environmental conditions like insufficient rain, which will make the plant less vulnerable to those kinds of opportunistic infections. Aucuba doesn't have many pest problems, and once symptoms manifest for a fungal disease, it would be too late to use a fungicide anyway since they are only preventative measures. (And not every fungicide ingredient works equally well for each potential fungal pathogen, so it would ideally need to be identified first based on early-stage symptoms.)
For now, see if you can determine if scale are abundant on the crapemyrtle's bark, or if SLF adults are present. It's late in the season for Crapemyrtle Aphids to be a problem, but you may be able to see evidence of their earlier presence if fallen leaves have shed skins stuck to them. I would not worry about any aphids this time of year if you do find some. Since many areas are still abnormally dry, the plants can be checked for watering needs to reduce any stress they are under from the drought that isn't over yet, unless the neighbor was irrigating them periodically this summer. A strong blast of hose water on the foliage and bark might also help remove some of the sooty mold and scale insects.
Miri
