Thanks for the question.

Very coincidentally I just trudged in from my backyard and noticed this same fungus on my diablo. I was not concerned nor should you be. Our shrubs will not die. Some of the leaves may fall off but not to any great extent. We could go out to our local gardening center and get some fungicide and spread that around. No harm in so doing. I really don’t like putting chemicals in my back yard so will not do so. A fungicide only prevents a fungus from germinating its spores. It does not kill the fungus per se. Keep in mind that this is a fungus that thrives under moist conditions, And as I just did, prune out some the lower branches of the diablo (they will grow back) so that there is better air circulation for the entire plant. Here is some information about powdery mildew and ninebark:

https://www.gardenguides.com/13428497-how-to-eliminate-the-powdery-mildew-from-diabolo-ninebarks.html

https://www.gertens.com/blog/controlling-powdery-mildew.html

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