Hi Vicki,

I'm sorry to hear about your shrubs.  From the photos, it looks like they are showing signs of powdery mildew, a common fungus that infects leaf surfaces.  A recent (2022) study showed that powdery mildew on ninebark was effectively controlled by commercial fungicides, an organic fungicide produced from giant knotweed (sold as Regalia), and by disinfestant products (for example, Zero-tol or potassium bicarbonate), although the disinfestant caused some damage to the leaves in high temperatures.  With any pesticide, be sure to read and follow all the label directions.

You can also trim off and destroy diseased branches if the infection is on a small area.  

Pruning your shrub to improve air circulation and being thorough in cleaning up leaves in the fall (because the fungus overwinters on leaves) can help reduce the severity of infections in future years.

I hope this is helpful!

Cordially,

John