Hi ,

I would start by thinning out all your old canes. This will help to allow more air flow which in turn, will help decrease the fungal transference. Certain varieties of ninebark are more susceptible to fungal issues.

http://plantclinic.cornell.edu/factsheets/willowpowderymildew.pdf

https://journals.ashs.org/hortsci/view/journals/hortsci/56/5/article-p532.xml

Spraying them with a baking soda solution is supposed to help, see the link. It looks bad but powdery mildew doesn't really hurt the plant.

Hope this helps, 

Sally Granath, St. Louis County MG