Thanks for writing. It sounds to me like your roses have powdery mildew, a very common disease. Help prevent powdery mildew by growing your roses in a spot with full sun and where air flows freely. Still air encourages the disease.
There's not a lot you can do to treat it. You can use a garden fungicide labeled for use on powdery mildew; it will stop the disease from spreading any farther so your roses can develop new, healthy growth. You might use the fungicide during periods of warm, humid weather --- spray as instructed on the packaging to help prevent the powdery mildew from coming back.
---Justin, Senior Garden Editor, BHG.com"