If the plant is of a manageable size, the easiest way to deal with sooty mold is to just wash it off. In itself, it is only cosmetic and harmless to the plant, unless it is heavy enough to interfere with photosynthesis.

What is of greater concern is the reason for the sooty mold in the first place. Sooty mold is a harmless fungal organism that develops only on honeydew or the sweet, sticky excretions of various sucking insects.........aphids, scale and mealy bugs tend to be the most common producers. You need to discover which of these is present nearby and then control them. The insects do not necessarily need to be on the crape myrtle....they could just as easily be on any overhanging plant and the honeydew is merely dripping down on to the crape. And since it takes a while for the sooty mold to develop on the honeydew, it could also be that the insect source is now long gone. But check thoroughly anyway.

gardengal, Thank you for the ideas. There is a tangle of Carolina Jessamine vine above this plant, although I've never seen secretion falling from it. I'll give it a spraying anyway.

As for trying to clean off my crape myrtle, could you think of any cleaning agent (maybe very mild solution of soapy stuff) I could mix with water to spray it?

IME, just spraying will not remove the sooty mold. You have to actually manually rub it off (it adheres to leaf surfaces pretty firmly). But maybe a mild Dawn dish soap solution would work. If it can get crude oil off of sea birds, it should have some effect on sooty mold!! Try it......let it sit for a while and then spray again with a strong stream of plain water.

Also, the honeydew that may be coming from overhanging plants would be unlikely to be very noticeable at the time. At best, you might notice a shininess to the leaves or a stickiness to any other flat surfaces but never any measurable 'drops' :-))

if its potted.. and coming from above.. move the pot ...

be careful with dawn ,,,, it is a detergent.. and that is why she said MILD ...

the usual culprits for honey dew is aphids or some a scale ... they should not be hard to find .. i doubt you would ever see it falling ...

we have heard of peeps whose cars are covered with the stuff from trees above ...

ken

Wanted to thank you all for trying to help. Seems as though photosynthesis was indeed being hampered. I used a spray bottle (straight water) on high steam and mature leaves came flying off. After a good shake the crape dropped about half its leaves and that’s when I found the mealy’s and white flies.

Bottom line, once all cleaned up, pruned, and insecticide applied, my prize bonsai crape looks like it did back in the winter - not a single leaf or errant stem ... a complete reset.

Fortunately the plant is strong otherwise and plenty of growing season left here - I feel pretty good about it. I scraped off the top half inch of topsoil, massaged in a little insecticide, and reapplied a layer of fresh soil. No signs of pests in surrounding plants, but treated anyway.

Hopefully that’ll take care of it.