With powdery mildew an ounce of prevention is worth a pound of cure. Hit them good with hydrogen peroxide solution. It's recommended to hit them again a week later but I'd hit them a gain a few days later. Realize doing this may bleach the leaves a little. Milk and water is useless. And to prevent this happening in the future? Invest in some Neem Oil. It is completely organic and wonderful stuff. Spray it faithfully and kiss your fungus problems good bye. Neem will also prevent bugs from laying their eggs. Remarkable product. Liquid copper also works for WPM but I wouldn't want to risk traces of copper sulfide in my dope. I'll use it on my roses, though. I keep editing this, but I'll add this: Before applying hydrogen peroxide spray them hard with plain water to wash as much of the spores off as you can. If they're outside garden hose them, let them dry and then apply the chemical.