Lemon juice
The ONLY way to minimize the stains is to dab lemon juice (you might mix it with water at first to test it's strength) on them & set them out in the sun. This should lighten them but probably won't take them out of the fabric completely. Mildew is the most heinous stain to get rid of! BTW, that info is straight off the stain remover "Shout"'s website on stain removal. Good luck!LaDonna