Does it bloom more than once a year? A very common pink rambler with mildew issues is Dorothy Perkins. Dorothy would mildew with or without air circulation so don't blame yourself! You can purchase a fungicide (Immunox comes ready mixed in a spray bottle) at Home Depot if the mildew is unsightly. Dorothy will bloom profusely and grow well despite the mildew so the problem's aesthetic. You can prune to shape the bush immediately after it blooms, pruning later in the year will reduce the number of flowers produced next spring.

It only blooms once. And it seems as though the powdery mildew prevents some of the flower buds from opening.