I would like to reduce the height of my crape myrtles by pruning back the crowns. Can this be done now, or what is the appropriate time/season to do this? Last year one of them had a mildew problem and I cut the affected portion out. This year the same area has stunted growth. Should I treat it or leave it?
If you need to reduce the height you can prune now. Crape myrtles bloom on new wood. However, they do not need a lot of pruning.
Prune out any stunted growth. The only way to prevent is by spraying and this is a lot of work. Some varieties are more susceptible than others. If you were ever to plant another, we'd urge you to select a cultivar bred with both disease resistance and improved hardiness. The U.S. National Arboretum has introduced many, most of which have Native American names.
This fungal disease is usually not lethal to crape myrtles but is certainly not attractive. Make sure the plant is in full sun and good air circulation to reduce humidity levels and allow proper plant spacing. Pruning for better air circulation also may help.
If severe, you can spray as soon as you notice the mildew. Check the label registration on fungicides for control information and timing. Fungicide treatments can help to protect new or un-infected foliage they are not curatives. It is up to you if you want to spray. You can monitor the situation this year.