It's unsightly but it won't kill the plant. It's most likely to turn up when it's hot and damp and the plants are close together. https://en.wikipedia.org/wiki/Powdery_mildew

was going to work on moving plants further apart…

but haven't noticed any white powder. I will go out in a bit when it cools down some outside and see if there is any. I have heard of powdery mildew on leaves of different plants, but not the stems.

plants, I have seen some of them "disfigure" and become disease prone. Best to thin.

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