Mildew, a type of mould, requires moist conditions to grow. The spores will grow on any surface from which they can get food, including synthetic fabrics like nylon. Mildew causes a discolouration of the surface and creates an odour. If it is allowed to thrive, it can eventually destroy a nylon backpack. It is best to remove mildew as soon as it appears to prevent the build-up of a stubborn stain.
Brush the mildew off the backpack. It is best to do this outside to avoid spreading the spores inside the house. Place the backpack outside on a sunny day to dry off the mould and remove the mouldy smell.
Wash the backpack as directed on the care label if the stain remains. The laundering process should remove the mildew from the nylon fabric.
Use a stain remover for stubborn stains. Mix equal parts of lemon juice and salt to form a paste. Cover the stain with the paste. Leave the backpack in the sun for about 15 minutes to bleach out the stain. Rinse the paste off the bag and let the bag air-dry. It is best to test a hidden area of the backpack for colour fastness before applying it to the stain.