...if the plants are still in veg then just treat them with Eagle 20 and it will rid them of the pm and if you're in flower use milk as has already been suggested, ...it works wonderfully as an organic fungicide that is safe to use throughout flower right up to harvest.
...btw, the milk won't rid your plants of pm in one spraying, it will need to be re-applied every 3 days or so until it stops coming back, ...in my own case that took nearly a month.
...do some research into the life-cycle of powdery mildew and you'll see the best way to deal with it is to prevent it through environmental control, ...meaning you need to learn what conditions promote pm and make sure you don't create those conditions in your grow, ...the spores are EVERYWHERE so if the environment is right, you get pm.
peace, bozo
...and like Uncle Pirate said, it ain't a big deal, ...the key in successfully dealing with ANY pest is to understand it's life-cycle so you can interrupt it.