After posting the above, I did a web search on the subject and came up with the following paragraph in a Nursery and Landscape Report out of Ontario. When I did a search last summer, I came up with very little, but now there seems to be a lot of talk on Ninebark and powdery mildew. Too bad. Sounds like a lot of effort must be put into controlling it.

Look for thick, white, powdery fungal growth on the newest leaves of Physocarpus opulifolius (especially Diablo). This is powdery mildew (likely Sphaerotheca sp.) that has been reported widely on our ornamental ninebark. This fungus sporulates quite profusely on Physocarpus and is difficult to control with fungicides once heavy sporulation is evident. Remove and destroy infested tips and continue to do so throughout the growing season while protecting new shoots with fungicides (where necessary).

Here is a link that might be useful: The report

I know that with perennials, drought-stressed plants are much more likely to develop powdery mildew, and I wonder if this is true of ninebarks as well. It seems counter-intuitive that dry conditions lead to more mildew, but it's apparently true.

"It seems counter-intuitive that dry conditions lead to more mildew, but it's apparently true"

Powdery Mildew does not really need moisture on the leaves to get estabished, it prefers dry leaves. The rain/irrigation wash the spores off the leaves...but high humidity it LOVES!!!!

No mold or mildew, but I am concerned my diablo may be sick.

I cut away a large dead branch a week ago. Now today I noticed a couple more dead.

I hope this isn't signs of a dying bush? It's 3 years old and has been doing fine. Bloomed beautifully last spring.

jasper