How do you get rid of it, once it is visible. It's attacking our crepe myrtles.
Queen Anne's County Maryland
Unfortunately, once you see powdery mildew it is too late to control it. Fungicides are used to prevent diseases and not cure them. Powdery mildew is not a serious disease but it is not aesthetically pleasing. Please see the following information from our website, http://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
For the future, if you plant crape myrtles make sure you choose a US National Arboretum hybrid that were breed to be powdery mildew resistant.