nothing you can do this year ...

total fall cleanup of infected leaves should help ...

a spray as simple as baking soda ... i THINK .... after all clean.. might help a recurrence next spring .... i MIGHT consider spraying the bark.. and mulch ....

or.. as a farmer would say ... next year will be better .... and you may never have the problem.. at this intensity again ...

air circulation is important.. but the root cause is high humidity during dark nights that stay warm .... and there isnt going to be much to do about warm nights ....

and all watering.. if any .. should be done early enough to allow going into darkness with dry leaves .... not that i feel established shrubs need any water at all .... in most cases ... especially natives ...

ken

Thanks, Ken. I did not clean up the fallen leaves very well last year, will definitely do so this Fall and spray the mulch and the bark then and again next Spring. It's interesting that only these two cultivars have it. It appears only on new tender growth and mostly toward the inside center of the shrub and near the ground, which tells me we're on the right track for some prevention.

lablover,
Yes, those two cultivars are infamous for powdery mildew.
Google the cultivars and see what has been reported for years.
The best cure for powdery mildew is WATER.
Spray it all off with your spray on your hose.
Water the shrubs deeply by laying hose at bottom of roots and turn on low for about 20 minutes for each one.
Water them more often than you have been and watch the mildew disappear.
Those particular cultivars need that.
Good Luck!

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

I've had a wave of powdery mildew attack my lilacs and my Engleman Ivy. I did some research and came across the suggestion to spray with a mixture of lowfat milk and water, in a 9-1 ratio of water to milk. I tried it, first on my ivy and then, when the powdery mildew stopped spreading on the ivy, I went ahead and sprayed my lilacs as well.

It doesn't get rid of the mildew, but it stops it from spreading. It worked for me, it's been three weeks and the mildew hasn't spread. Next year I'll probably spray in early August so it doesn't get a chance to get started.

Being milk and water, it's harmless. The research recommended using skim milk, to keep the odor down, and the sprayer from getting clogged.

Good luck.

-jwt

I posted a message last year (2010) saying that my new Coppertina was covered with powdery mildew, even after rigorous spraying. I cut it down to @2 1/2 feet from the ground. This April (2011) I transplanted it from the corner of the yard (surrounded on 2 sides by fence) to close to the middle of the yard where it gets sun all day. Then sprayed it for 8 days straight with baking soda, hydrogen peroxide and soap. That seemed to control the p.m. More spraying with straight H2O2 every 4-5 days. Now it's almost July and p.m. is hardly there anymore, but must keep spraying from time to time. The plant is thriving!!

All Ninebarks are susceptible to PM, not just those cultivars by the way.

Humidity and dry soil are the ripe conditions so it will be worse in some years than others. Good amount of sun with moist soil conditions are preferred.

I had Diablo and had to cut back to 3" stubs. They sprouted back up without PM.