Experts and BHG readers answer.
Mildew on Roses
This sounds like Powdery Mildew which is a very common plant ailment that often doesn't do too much damage, but can become a problem. The only way to combat Powdery Mildew is by using a Rose Fungicide every 7 to 10 days during the spring. It's generally only a problem during spring weather and goes away when the climate gets hotter and drier."