My purple cone flowers are starting to bloom. I have seen some white powder on this plant stem

[arachadillo]

7. Cone flowers

I agree with the thinning, sometimes high density flower patches promote powdery mildew just because all the parts of the plants are not getting sufficient sunlight.

Over watering the plants also sometimes causes the problem as well as having weeds or other plant stuff near the garden.

Finally, if the problem is way to big to deal with and the garden patch growing the flowers is important for any reason, including aesthetics, there are a handful of organic fungicides that can be used for prevention. As long as the fungicide is for any daisy or more generally any garden perennial, it should work good.
http://greennature.com/article267.html