The white fungus growing on your ninebark (Physocarpus opulifolius), appears to be powdery mildew. This common fungal disease displays a white, powdery growth on plant surfaces such as leaves and flowers. Powdery mildew is mostly a cosmetic problem on many plants; however, on ninebark it can cause serious leaf loss and even branch tip dieback.

Powdery mildews are favored by warm days and cool nights and moderate temperatures (68° to 86°F). At leaf temperatures above 90°F, some mildew spores and colonies (infections) are killed. Shade or low light intensities also increase the development of this disease. Good air circulation around plants and proper pruning within the plant will help reduce the incidence of powdery mildew. This disease spreads when wind and splashing rain disperse the fungal spores onto uninfected leaves.

Spraying the foliage with a fungicide will help prevent the spread onto healthy leaves, but it will not make the infected white leaves look healthy again. Fungicides are available at your local garden center.

Rake up and remove the fallen and diseased leaves to reduce the odds of re-infection every time the weather favors disease. And for sure do this at the end of the growing season after the leaves drop. 

Your ninebarks appear to be quite dense with foliage. It might help to prune out one-third of the biggest, oldest shoots right to the ground each year after your ninebarks bloom. This will thin out overly dense growth and increase air flow through the plant, which will help dry the leaves and take away the dampness the spores need to germinate.

Avoid fertilizer, especially ones high in nitrogen. That also encourages powdery mildew.

The links below should be helpful:

https://hort.extension.wisc.edu/articles/powdery-mildew-trees-shrubs/

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