What a Mess!
This crapemyrtle has three different pest issues happening at the same time. Should a plant in this condition be doctored or thrown out? Fair question. First, let’s examine the issues, starting with sooty mold. Sooty mold is a fungus that produces dark, microscopic threads in such abundance that the surfaces of leaves, stems and twigs can appear black. Sooty mold on a landscape plant is usually an indication that a sucking insect such as aphids, soft scales, mealybugs or whiteflies is present. These insects excrete excess sugar in the form of “honeydew”, which in turn is an excellent growth medium for sooty mold. In the case of crapemyrtle, the offending insect is usually aphids, and aphids were found on this particular sample. (Unfortunately, crapemyrtle bark scale was detected in NC in 2016, so crapemyrtles with sooty mold should now be inspected for that insect as well.) Controlling aphids on crapemyrtles is difficult, and if you happen to have a cultivar that is highly susceptible, control efforts may not be worth your time. In addition, aphids tend to attract the attention of beneficial insects such as lady beetles and green lacewings, which often do a nice job of keeping the aphid population in check. Frequent sprays meant to control the aphids will also have an effect on the beneficials. See https://www.ces.ncsu.edu/
depts/ent/notes/O&T/shrubs/ note31/note31.html for more information.
That’s two problems – aphids and the resulting sooty mold. The third problem is cercospora leaf spot, a fungal disease that is quite common on crapemyrtle, but more severe on some cultivars than others. The bright patches of red, yellow and orange are caused by a toxin that is produced by the fungus. Other than premature fall defoliation, cercospora leaf spot causes little harm to infected crapemyrtles, and fungicide sprays are not recommended. However, raking up and disposing of the leaves as they fall – a process that could drag out from September well into November – can reduce the presence of the pathogen on that site.
So – should this particular crapemyrtle be dug up and disposed of? That’s up to the homeowner or landscaper, of course. But if the appearance of a plant is not acceptable, and the only recourse is frequent pesticide applications year after year, moving on to something else is a good idea.