Mold and mildew are fungi, and the stains we associate with them, typically pink, yellow, purple, or black, are byproducts of their digestive process. The compounds they produce can diffuse into a textile and discolor it in a way that can’t be removed simply by washing it.Fungal spores will grow in any warm, moist location where they can find a food source. Keeping a fabric clean and removing any mold or mildew with diluted household bleach (only for bleach cleanable fabrics and only in the recommended ratio) will reduce the number of spores and food sources available to cause problems. Choosing designs that allow water to run off of a surface or escape from inside of a cushion will also help prevent mold and mildew from growing.
A mildew resistant fabric has not necessarily undergone any specific chemical treatments. The fibers themselves may inherently resist mold, mildew, and bacteria growth. Unlike natural cellulose and protein fibers, petroleum-based products (polyester, nylon, polyethylene, and polypropylene, for example) create environments that do not encourage the growth of mold, mildew, or bacteria. Although a resistant fabric will not prevent such growth in the right conditions, it can minimize the severity of potential problems in hot and humid locations.