To remove banana stains from fabric or clothes then have a look at the following
Wash the item in the hottest wash that the item can bear. Avoid using soap as that can set the stain. You can also soak the item in warm water with a little enzyme prewash stain remover for about half an hour. If the stain is really old then you may have to use a mild laundry bleach, If your garment allows it.