A spraying every few weeks of dilute milk (one part skim milk to five parts water) will keep powdery mildew at bay much better than does sulpher or commercial fungicides. But afflicted clumps should probably be moved to an airier location, & foliage clipped entirely to the ground to start over. Mildewed foliage should be burnt or bagged to discard, not composted, as the spoors can survive compost heat.Perhaps that will prove useful!
There was a recent post on mildew, asking how to treat it. Paghat has this to say: