I was told there was good results for mildew using a baking powder solution. If you know the formula, I would appreciate it. It also had small amount of dish soap.
Powdery Mildew on Ninebark Shrubs
Certified GKH Gardening Expert
Recipe: 3 tablespoons baking soda plus 1 tablespoon of Murphy's oil soap, dish soap, or a lightweight horticultural oil in 1 gallon of warm water. Mix well and let cool. Shake well just prior to spraying. Apply this spray (making sure to get the undersides of leaves) every 7 to 10 days until the powdery mildew is gone.