Answer: Powdery mildew is a fungal disease that attacks plants when environmental conditions are right (warm days, cool nights, some humidity) and air circulation is poor. Ninebarks are known to contract mildew when conditions are right. Although Diablo is actually considered resistant to this problem, in a bad year it too may suffer infection. But, even though it suffered this season, it may not be affected next year as it really depends on the weather each year.
You can help avoid powdery mildew by giving plants the required exposure to sunshine, and plenty of elbow room so there's good air circulation all around them. If your ninebarks are thick and overgrown, you might want to consider some judicious pruning next spring to thin them out and improve air circulation to the interior of the shrub.
Also do a thorough clean up around the shrubs this fall and remove fallen foliage and any prunings to limit carryover to next year. (Powdery mildew spores do not overwinter in the soil.)
It is really too late in the season to try treating the plants now. This is a problem that should be treated at the very first sign of infection; waiting until the problem is severe makes treatment that much more difficult. Keep a vigilant eye next season and, if you are going to spray, please work with your local county extension to determine what to use and how/when is best to apply it for maximum results. Generally speaking, you would want to take action at the first appearance of the problem and continue to spray preventively.
Q&A Library Searching Tips