Mold and mildew are happy to find a home in organic materials like fabric, paper and leather. If you live in a humid environment or a home without proper airflow, you're bound to come across some sooner or later. They can damage fabrics by causing discoloration or even eating through the material.
Fortunately, a paste of lemon and salt is a great spot treatment for tough mildew stains. Simply apply the paste to the stain, set the item in the sun to dry and then wash as you normally would -- preferably in hot water to kill any remaining mold spores.