I have had relatively good luck ridding plants of powdery mildew by spraying them with a milk solution.
One part milk to 9 parts water.Measure does not have to be exact. I have found whole milk does a better job than skim and it needs more than one application 2-3 does it for me.
I have used this mostly on phlox and a few other plants ,I have goldenrod only in the wild areas of the garden and they do not get much attention.
