One thing I've noticed about powdery mildew is that as soon as we hit any sort of drought, it starts it's onslaught. It's almost as if it waits until the plant is a little stressed and then BAM, it's on.
I also have a coreopsis (Zagreb) that gets PM every year after flowering. I think this is due to it being in too much shade but the PM doesn't do anything other than make it look ugly. I cut most of it down a few weeks ago as an experiment. I think it will come back fine though.
