This sounds like powdery mildew, a fungal disease. It is common in warm dry climates when the relative humidity is high, as it is now. It's usually a problem when plants are crowded and damp, shaded areas. The fungus grows on plant surfaces, shading leaves and interfering with photosynthesis. Severely infected leaves eventually can discolor, deform, yellow and drop. At this time of year, you may wish to remove infected leaves or rake them up after they drop later this fall. Doing so will reduce a source of infection next year. To reduce humidity around the plants and decrease infection, avoid overhead watering. When you prune your roses next spring, thin out dense growth. This will improve air circulation in and around the plant, which can reduce chances of infection.

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