Hi, 

Powdery mildew is very common for crapemyrtle this time of year and the same with Japanese beetles.  The good news is that they don't seem to negatively impact the health of the plant overall beyond being nuisances. 

You can try horticultural oil to help with the powdery mildew but you will need to check the label for the maximum temperature that you can use it so it doesn't burn the foliage. 

You can hid pick off the beetles and drop them into a cup or bottle of soap water. We don't recommend using any pesticides for them since it would be harmful to other beneficial insects as well. 

Let us know if you have further questions. 

Emily