Crepe Myrtle with Powdery Mildew
This fungal disease is usually not lethal to crape myrtles but is certainly not attractive. http://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
Control begins by selecting powdery mildew resistant varieties and you should consider for the future. You will have to decide.
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.
Look at the list from the National Arboretum for more information on resistant varieties http://www.usna.usda.gov/Gardens/faqs/crapemyrtlefaq2.html#How%20hardy%20are
and Clemson http://www.clemson.edu/extension/hgic/pests/plant_pests/trees/hgic2002.html