My crape myrtle has white and black powder. How do I make it healthy?

The white powder is probably powdery mildew. This fungus grows best in damp conditions. The black powder is sooty mold which is growing on honeydew from sap-sucking insects. Both problems can be treated.

Related Videos:
Treating Crape Myrtle Bark Scale
Identifying Crape Myrtle Bark Scale
Mixing and Applying Fungicide

Related Resources:
Crape Myrtle Bark Scale Identification and Control
Sooty Molds