This fungus forms a powdery white or grayish coating on the upper surface of young leaves and sometimes on the buds. Infected leaves crumple and become distorted.
Unlike blackspot, wet conditions actually inhibit the development of powdery mildew. It can not reproduce in water. It thrives during high humidity but forms on dry leaves. Warm dry days, cool dry nights are ideal for powdery mildew.
One of the best ways to avoid powdery mildew is to keep things as airy as possible. Roses planted too close to a wall may not get enough airflow. Prune away crossing canes and open the center of the bush to allow sunlight and airflow.
Also, spraying the foliage with a mixture of 1 T. baking soda per 1 gallon of water can be effective.
See blackspot for other treatments of powdery mildew.