White spots on crapemyrtle bark is an insect called Crapemyrtle Bark Scale, and they are treatable. The insects suck plant sap, and they excrete a clear, sugary waste liquid called honeydew. This sticks to surfaces below the insects, and on top of it, sooty mold (the black residue) can grow. Sooty mold does not infect plants or cause disease, but it can be an eyesore and take a while to weather off on its own. Once the insects producing honeydew are reduced in number, the sooty mold will gradually go away.

If the tree had a disease (nothing in what's left of it indicates that it did), then it might have been unrelated to the scale issue. One possible cause of dieback is Botryosphaeria canker, which can infect trees and shrubs suffering from drought stress or other weather-related injuries. We have seen several examples of mature crapemyrtles that had branch dieback this year from last year's drought coupled with a winter cold snap or two.

The web page linked above, plus our Introduction to Scale Insects page, will include management options. Essentially, they involve physical removal (gentle scrubbing or pruning as a last resort) or chemical treatment with an insecticide, some of which will require the hiring of a professional to apply. There are beneficial insects that can eat the scale and reduce their numbers, and some management options will interfere with their help, so it's best to see if they are present before taking action. An example of one, a ladybug larva that looks nearly identical to the scale (only larger and mobile) is pictured in the Crapemyrtle Bark Scale page.

You did not need to prune the plant back, and doing so unfortunately may have increased its stress, especially since the pruning cuts were not made in the right location and are too angled. While crapemyrtle plants that are otherwise healthy can rebound reasonably well from heavy pruning, drastic trimming can result in poor branch attachment as they regrow, which makes them more vulnerable to breakage or storm damage as those new stems mature. It might be simpler at this point to replace the plant instead, or prune it even further in order to get the pruning cuts in a better location so regrowth is less susceptible to problems. You may want to hire a certified arborist or licensed tree expert to help you give the tree corrective pruning.

Mulch is not a contributing factor to the tree's pest outbreak, and it helps to moderate soil temperature and reduce moisture loss from evaporation, so we suggest putting some mulch back in place. That said, it should not be mounded or piled against the base of the trunk, so keep the layer flat and only about 2-3 inches thick. The semi-elastic green ties on the trunk need to be removed so they don't constrict trunk growth. (They are designed to break when they stretch too much, but you don't want to risk that they don't stretch enough and begin to damage the bark or wood.)

Miri

You're welcome.