Answer: Triforine is a locally systemic fungicide used for black spot, powdery mildew, and rust of roses, powdery mildew on azalea, begonias, delphinum, kalanchoe, plane tree, calendula, crepe myrtle, dahlia, euonymous, Jerusalem thorn, lilac, phlox, snapdragons, Photinias, and zinnias, rusts on aster, carnation and oxalis, petal blight on azaleas and rhododendron and Entomosporium leaf spot on Photinia. It sounds to me as though you're doing all you can to control powdery mildew on your Ninebarks. Pruning out the most affected branches will open the plants up to better air circulation which should also help. Best wishes with your ninebarks!
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