This has happened to me every year. My petunias will be blooming like crazy, then all of a sudden the flowers will start getting white spots and it happens so fast that you can almost watch it before your eyes. It doesn't kill the plant. It will happen to all of them (different varieties and planting locations). Nobody I have talked to have heard of or seen this.
Heidi in Minnesota
i might be wrong but sounds like powdery mildew try spraying them with soap and water and rub the spots off the flowers will love it and it might work well for you just don't use antibactiral soap ok