Try this recipe for roses: https://www.thespruce.com/baking-soda-for-controlling-powdery-mildew-1402520
There are sprays to use that any garden store would carry. if you catch it early you can spay it with milk. Spray all leaves, top and bottom.
the milk spray works well cover all over safest on plants and soil
Make sure your roses are no in a spot, where it is cluttered with other plants. Roses (like tea roses) like air circulation.