It's best to wait until the leaves are dry, midmorning through afternoon, since we can unwittingly spread diseases by touching wet plants. If you notice signs of disease, remove affected leaves and apply a preventive spray as directed on product label. In the fall, remove all fallen leaves on the ground below the plants, and dispose of them. In late winter or early spring, prune plants and apply horticultural oil to smother disease organisms that may have overwintered on branches. Spread a fresh layer of mulch to cover any disease spores that might be on the surface of the old mulch.
Roses are so popular and widespread that many sprays are available to control their diseases.
Remove all winter prunings, a common source of reinfection. In early spring before growth begins, spray a combination of dormant oil and lime sulfur. Powdery mildew causes a grayish white fuzz on new leaves and flower buds. It spreads on dry leaves, usually during periods of low rainfall and warm, humid nights. Use a spray containing potassium bicarbonate, basically baking soda but in a form that is more plant friendly.