Mildew on roses
How do I get rid of mildew on the leaves of roses?
Powdery mildew is caused by insufficient water during hot and dry weather and by poor air circulation. There are commercial products for treating mildew on plants. Spray in the early morning once a week until the mildew is gone. There are numerous DIY formulations with baking soda, dish soap, etc but I can't vouch for their effectiveness. Here is the American Rose Society page on powdery mildew: