Its powdery mildew, ninebarks are extremely sucspectible to it. Its not really a big deal, its mainly an aesthic issue.

You could spray with a fungicide every 10-14 days and that will limit the spread. You can prune any severe infections. During cool moist nights, followed by dry windy days really accelerate the spread.

I just fought with 7 Diablos last year and this year and it seems to finally be under control.

I think it is a fungus. But on the underside of the leaf on picture 1 it looks so thick. I would see first by feeling and squishing it if it is a bug. If you just bought the plant, I would exchange it for another. Maybe you are having wet weather with the conditions just right for fungus. There are spores in the air just wanting the right place to settle in. I would expect that your shrub will not infect others but that it is stressed.

Possibly powdery mildew. Keep water off the leaves and allow the soil to dry some. There are commercial product available to control this but improvements in cultural practices is sufficient.

hey I have the same issue! Did you get a solution to fix the problem?

Here is a link that might be useful: pictures

Oh and by the way the powedery mildew is host specific.

My diablo ninebarks all had it for 2 years, the freeman maples, snowmound spirea, autumn blaze pears, witchhazel, fine wine weigela and korean lilac were all in the vicinity and didn't get the powdery mildew.

Some years the ninebarks will get it worse than other years. Just keep up with the fungicide and clean up leaves in the fall. Prune out badly infested areas (full white coating on a leaf).

My Coppertina was planted last year, had beautiful flowers and seed pods. This year it developed the mildew; was too stressed to flower. I've been snipping off affected leaves and branches, spraying every week. Finally I pruned it 1/3 down, hoping it will grow healthy branches. But the mildew is still there. It has been a wet summer. But I'm really worried that the mildew will kill the bush.

Moisture has little to nothing to do with powdery mildew, unlike other plant pathogenic fungal diseases. It doesn't require moisture in order to infect. As a matter of fact, a good method of controlling the disease before it gets too bad is to spray your plant to wash off the spores.

The primary problem is the choice of plant material. Ninebark is simply a terribly susceptible plant. I never recommend this plant, nor have I used it in a landscape design in many years for this reason. It's a shame, too, because they are so pretty.

It just doesn't make sense to bring something into the yard that requires such care just to keep it alive.

Here in the intermountain west, they do pretty well as PM isn't as prevalent. I have three and maybe 10 leaves on one plant (closest to lawn overspray) get it per year. Easy enough to pinch off and squirt neem once in a while.

Dan

Here in New England and my experience is that out of 7 very healthy plants, only one had a serious case of powdery mildew - not enough of an issue to avoid ninebarks.