White Powder on My Crape Myrtle
This looks like powdery mildew, a common fungal disease on the foliage. This can weaken the plant but does not kill it.
Here is more about it https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
If you have a susceptible variety of crape myrtle, this could be a yearly problem, in which case replace it. You can also try spraying at the first hint of powdery mildew to prevent build-up of disease. But consider whether you want to do that when there are plenty of good disease-resistant varieties to be found.
What should I do? The tree is getting tall. Should I wait until the leaves die to dig it up, or should I try to get rid of it now? I have a lot of plants that may be affected nearby.
The powdery mildew will not spread to other species of plants (only crape myrtle.) Powdery mildew fungus is very specialized and there is a different type of powdery mildew for each species of plant.
You can get rid of the tree whenever you want. If it has never had a problem with powdery mildew before, it should be worth keeping. You can prune off the worst tips of spray with a fungicide to stop its spread this year (it will not remove the powdery mildew that is already there.)
Read the link in the previous post to understand it better.