Just an FYI. Get yourself a bag of garden sulphur and a paintbrush.
Dust the leaves top and bottom 3 times a week, leaving the sulphur on the leaves for atleast 24 hours before spritzing off with water.
After a couple weeks, you shouldn't see the powdery devil mildew again. Dusting a light sulphur coating onto your plants during veg can help mildew ridden strains stay healthy AND mildew free.
Make sure that you don't do this during flower unless necessary, and try to not get any sulphur on your buds.
This remedy is best used for the veg stage only. It's also 100% organic.
Happy growing.
Garden sulphur can also deter mites, and other pests. However, almost nothing other than a nuclear bomb will kill em....mites that is.