Hi Doris, here's a great article on how to prevent and stop powdery mildew on roses - https://homeguides.sfgate.com/rid-powdery-mildew-rose-bushes-31948.html
Roses needs lots of sunshine to prevent mildew. Also, try watering them at the base, not from the top with a hose. If you get lots of rain, you need to move them where they will have sun most of every day. I also pinch off any leaf if I see any mildew and take it away so it won't spread. Also keep any mildew off weeds or any other plants in the area.
Hi Doris, here's another post that might help. https://www.gardeningknowhow.com/ornamental/flowers/roses/roses-powdery-mildew.htm