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It looks like your ninebark plants have powdery mildew. It appears from the articles linked below that powdery mildew can be more of a problem for ninebark plants than some other plants. You might want to consider fungicide applications. Fungicide active ingredients that could be effective are mentioned in the articles. Always read and follow all label instructions when using pesticide products. The label is the law!
Powdery Mildew – Trees & Shrubs from U. of Wisconsin-Madison Extension
Powdery mildew on trees and shrubs from U. of Minnesota Extension
Ninebark (Physocarpus spp.)-Powdery Mildew from Pacific Northwest Pest Management Handbooks
Please let me know if you have further questions after reviewing the articles!