Or you could move them. Also, I've read that adding one tablespoon of milk (any kind - skim, regular, etc.) to one gallon of water, and spraying the foliage once or twice a week for a few weeks, will get rid of the mildew. It couldn't hurt to try it if you don't want to move the plants - at least the milk mixture won't do any harm if it gets on your herbs. Of course, by now you may have already ripped out the salvia, seeing as how you posted this nearly a month ago and didn't get a reply.