could you favor us with a picture???

mildew is easily knocked down ... in a variety of ways... if in fact ... it is bad .. enough to react ... remedies go from baking soda in water.. to chemical ....

my worst mildew problem.. happened when i was watering in the evening.. wetting the whole plant ... going into darkness.. that is when the mildew grows ...

even if you stop it.. the 'look' might not go away .. until leaf drop ...

often.. the conditions requisite to the problem.. simply resolve themselves ...

we need more info to guide you.. including that pic

ken

See the attached link for a discussion on this subject - it's a bit long but very thorough :-) One of the most interesting aspects I gleaned from this study is that ninebarks have their very own species of PM!! Who knew?

IME, once established, ninebarks need minimal watering and I would avoid any overhead application. As with other PM issues, good air circulation and preventative fungicidal spraying will help also.

Here is a link that might be useful: Physocarpus and powdery mildew