I'm currently 3 months (give or take a week) into my outdoor grow and so far I have experienced and conquered several problems with my plants. I now am dealing with what I believe to be powdery mildew, If you think you have it yourself there are several remedies that I have read about.
1. Mixture of Baking soda (1 tablespoon), Dish soap (1 tablespoon) and approx 1 gallon of water. Spray on affected areas once a week.
2. Neem oil. (this stuff can usually be found in any garden center)
3. Milk and water mixture approx 10% milk and 90% water on affected areas
These are just a couple I found. I decided to go with a "green" fungicide that I purchased from a hardware store. Many plants can have problems with Mildew and fungus so thankfully most garden centers will carry products and have advice on how to deal with it. From what I gather, once affected with this stuff its there to stay but the mentioned solutions will keep it in check. Prevent it from spreading and reduce the infection. I would suggest also to remove any highly contaminated leaves entirely.