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.