The white fungus is called powdery mildew. Powdery mildew is characterized by spots or patches of white to grayish, talcum-powder-like growth on the upper side of the leaves. This is because the fungus does not need the presence of water on the leaf surface for infection to occur but the relative humidity of the air does need to be high for spore germination. Remove and destroy all infected leaves.  Prune overcrowded shrubs  to help increase air circulation and reduce the relative humidity. n Minnesota, powdery mildew increases in the spring and fall, when cool nighttime temperatures are followed by warm daytime temperatures. 
 Fungicides like Thiophanate methyl,Chlorothalonil, Sulfur (not for sulfur sensitive plants like viburnum), Potassium bicarbonate and neem oil should be used Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, and timing of applications.  Your plant may be a ninebark. 
https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs#using-fungicides-1584911