There are a few options for white mildew also known as "powdery mildew". This sulfur-based product worked very well for me during one of my outdoor seasons after a period of rain. After baking in the California sunshine in dry conditions for so long, sometimes rain can work against you. This stuff knocked out the powdery mildew issue in a matter of days. Less than a week it was gone. I had it pretty bad because the wind was spreading it. Luckily it was during veg season so no flowering was going on for the mold to get stuck in.
It's totally safe and organic and it works great. I tried a few things that were recommended to me when I had this problem and after using different ones on different plants, this was the winner. Look for it at your garden store, they have many products so make sure you get the sulfur based fungicide spray.