Funny, I have it on one branch of some winterberry hollies…which I have never seen before. Very weird. Does it look anything like any of these? http://ohioline.osu.edu/hyg-fact/3000/3047.html On my holly the mildew is white and almost crusty, yet when I rub it with my fingers it doesn’t dislodge…just stays put. Like the leaf was painted white or dipped in a sugary crust. Apparently mildew from one species doesn’t care about other kinds of plants (so lilac mildew is not interested in your Physocarpus, and vice versa). But what I wonder is why it has come back again. Is you plant in a good airy light spot?