Mildew
Mildew is a live, growing organism that can attach to cellulosic – cotton, linen, ramie, rayon – and hair – silk, wool – fibers. Mildew eats the fibers, damaging and weakening the fabric, and can leave those nasty black spots.

To remove mildew, first shake or brush the item outdoors to prevent spreading the mildew inside your home. Spot the stains with a heavy-duty liquid detergent. Allow to work for at least 15 minutes. Then wash the fabric in the hottest water suitable for the material with a laundry powder (as they contain Alkali Salts). Use chlorine bleach on white cottons to help restore whiteness. Drycleaning will kill the mildew, but may not remove the black spots, especially if the mildew has been present for some time.