How do I deal with mildew in a damp dirt crawl space?

I have a dirt crawl space under my house, about 800 square ft, average height 2 ft. It's damp all the time. Is there a way to treat the soil so mildew will not grow? If not should I cover the entire area with plastic even though the mildew will probably continue to grow underneath it. Because of the way the house was built ( the footer is only two courses of block below grade ) in the convergence of two hills, all moisture drains toward the house. I've put in drain tile which helped a little. Any suggestions would be appreciated.