Ninebark has known susceptibility to powdery mildew. Giving shrubs room for air movement can decrease it. Once established it is difficult to eliminate. Although it can be visibly unwelcome it typically does not impact the health of the plant. If you want to fully eliminate it is suggested to plant shrubs that are resistant. if you are still interested in ninebark, Little Devil and Summer Wine are resistant varieties. Below is a link with detailed information on powdery mildew.

https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs#:~:text=Powdery%20mildew%20does%20not%20cause,and%20purple%2Dleaved%20ninebark%20shrubs.