Crapemyrtle Bark Scale is the particular scale insect present, and the black residue is sooty mold, a fungus commonly associated with sap-feeding insects (it grows on their honeydew waste residue). Although it is a fungus, it does not infect plants and merely grows on any surface where honeydew has built up. It will weather off on its own in time and, for late-season coatings on deciduous plants (that will shortly be dropping their leaves), it does not need to be removed. The linked pages above provide more information.

For the future, do not use ammonia or dish soap on plants (and don't use them together). Home remedies come with many risks for the environment and the plant, and usually do not work well or damage the plant more than the original issue would. They are not recommended, and only registered pesticides (when they are warranted) should be used. Always follow label directions for the lowest-risk use of the product.

One of the lowest-toxicity options for treating Crapemyrtle Bark Scale is horticultural oil, either as a "summer" rate or "dormant oil," both of which are discussed in the linked page. There are several different kinds of oils which fall under the category of a horticultural oil; neem oil is a widely-used example. These are contact-kill pesticides, meaning they must directly contact the insect in order to work, and scale insects are largely protected from direct sprays by their waxy body covering. Oil can work to suppress them, but only if used repeatedly and, ideally, if used while crawlers are out, which is the most vulnerable life stage because they have no waxy shield over their bodies yet. With at least two generations per year, there will be two crawler periods per year, and any spray used will likely need several applications to control these multiple generations.

There are also natural predators that consume the scale, and a spray of oil (or any other topical pesticide) might affect them as well. There are ways to reduce this risk, including using dormant oil during late autumn or winter when the beneficial insects will not be active. Volck oil is a type of dormant oil and is used only once foliage is gone, or else its higher concentration risks "burning" the leaves.

Malathion is a potent insecticide compared to horticultural oils and other lower-toxicity options. It is not recommended for any home garden pest because there are less risky alternatives (both risk for people and risk for pollinators and harmless insects). It falls under the chemical category of organophosphates, of which few, if any, modern insecticides still utilize (some ingredients have already been banned over the years). It should not be needed for this situation and, if a systemic (plant-absorbed) insecticide were needed to suppress a high scale population, a certified pesticide applicator would need to be hired to apply it. (If using a neonicotinoid-class product, which is illegal for uncertified users to apply in Maryland.)

When the scale numbers are lower and the honeydew they produce wanes or rinses off in rain, the sooty mold will run out of food and go away on its own. Some horticultural oil pesticides are labeled for treating sooty mold as well, even though their primary target is insects. We don't know how long it will take for dead/dying mold to flake off the foliage and disappear if you were to try spraying it with the oil.

Miri