This is a fungal disease called Powdery Mildew; one of the most common diseases of roses, peonies, phlox (and many other plants). Moderate temperatures of 60° to 80°F and shady conditions are the most favorable for powdery mildew development, and will spread. The fungal spores are spread by wind. The good news is, you can manage this disease; sometimes fungicides are needed. Actinovate and Neem oil are two organic products that can help you with eliminating the fungus. Be sure to pick up any leaves and throw them out – do not put them in the compost pile. Also – do not water the leaves. Only water at the base of the plant. Do your best this year to save the plant, and if next year is more favorable the situation should be better.