During the off-season, backpacks might simply get thrown in a pantry, closet or even somewhere in the basement. When a backpack is left to lie around, especially in a space that is damp, dark and not well ventilated, mold can definitely become an issue. However, when you find mold on a backpack, this does not necessarily mean that you have to toss it and purchase a new one. You might be able to remove the mold and the mold stains.
Shake the backpack out outside, well away from your home. This will cause mold spores to spread, so protect yourself with a face mask.
Brush down the backpack with a thick-bristled brush. Stay outside to avoid spreading mold spores inside the house.
Create a mixture with 1 qt. of water and 2 tbsp. of either bleach or lemon juice. Be sure to double-check that the fabric of the backpack is colorfast if you use bleach; test the mixture in an inconspicuous spot.
Wipe down the backpack thoroughly with this solution using a rough sponge. Scrub vigorously areas that are stained by mold.
Rinse the backpack off in a utility tub or with a hose outside.
Lay the backpack in the sun to dry. Turn it over frequently so the sun gets to all sides. This will help to eliminate any remaining mold spores.