May often be associated both with drought stress and with fog or dew in later summer. We often have both here, with susceptible kinds of plants mildewing conspicuously after the season has been in effect for awhile. Some mildew spores travel with their own moisture packet, making germination on dry leaf surfaces possible. Blowing around and trying to stick to leaves during pelting rains is probably much more hazardous than appearing during dry conditions.

Not just humid nights, but dry days also cause mildew. June and August are horrible times of the year here in WI for mildew. Can't say with certainty that those 2 are more susceptible or not, but ALL Physocarpus varieties I've dealt with are horribly susceptible to mildew. Dart's Gold, Gold Nugget, Diablo, Summer Wine, Center Glow, Ruby Spice, straight species, and a dwarf green variety that I can't recall the name of have all gotten mildew in nursery conditions as well as landscape conditions. Green Cure is an organic fungicide that works great as a preventative spray. Using a fine spray from the hose and spraying until water runs off the leaves every morning will help cut down on the problem, and as already stated proper watering to keep the plants hydrated. The Plant Geek www.confessionsofaplantgeek.blogspot.com

That's interesting about there being plant specific... Nature is fun. I was just going to the extreme to stress the point that I think the important word would be "delay" vs "prevent" with pm. I have one planting of an older variant of phlox and pm always gets it early right as it starts to bloom. I remove the entire plant to the ground fairly early each year.... But it always hits the exact same time the next year. I have no other phlox and I live in a garden desert in my subdivision... I don't think a single home I pass in and out of the neighborhood has anything besides the original landscaped plants.... Non being phlox. The only other perennial that gets pm for me was an old peony... Which got the axe because of it. And then of course the late season squash get it. so to the original poster... Given your circumstances ..... And this isn't a plant that you are passionate about enough to put up with tight cultural practices.... And ...You would only being delaying the inevitable maybe a bit later in the season....(as others bring up.)....Looking into a replacement species would probably work better for you.

because that is what they do as they shut down for winter ... it is sept.. and in a few weeks you can just cut them to the ground.. and stop looking at them ... id wait until its a bit colder.. just to insure i didnt stimulate new growth with only a month or so before frost ... that would just be a waste of stored energy it needs for next spring ... you have repeatedly noted drought in your garden this season... so cecily is spot on ... stress is stress.. and it always amazes me.. how bugs and disease.. can find already stressed plants to attack .. ken