Answer: Yes, it is quite possibly powdery mildew. Although ninebark is usually problem free, this fungal infection can appear under certain weather conditions. It can also spread from plant to plant, especially if they are crowded or located in a place with poor air circulation. Clean up any fallen foliage and pruning debris to limit sources of reinfection. It can be helpful to wash them off with plain water from the garden hose.
After washing them off, you can also treat with fungicides, if you use a fungicide be careful to follow the label directions exactly. Fungicides are usually most effective if applied preventively or at the very first sign of an outbreak.
If the plants defoliate, they should regrow new leaves and recover. This should not be lethal -- although they may look a bit ugly this year. It may not recur next year as it is weather related, but be sure to do a thorough clean up this fall to limit sources of reinfection.
For a more precise diagnosis you might wish to consult with your local county extension. If a chemical control is needed, they will also have the most up to date information on what to use and how/when is best to apply for maximum results.
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