Deborah, I agree ... if you can post a photo or two showing the white stuff at the base of the flower head it might be easily identified. Meanwhile, here's a link to images of powdery mildew:
https://www.google.com/search?... Most images show the mildew on the foliage but if you sroll down the page a bit there are some roses or other flowers with mildew at the flowers.
And, in case it's mealybugs, here's are links with photo's for those critters:
http://gardeningnaturallywithc...
http://www.organicgardening.co...
