Mildew---a fungus---is a form of mould that thrives on damp surfaces in areas with high humidity that are void of sunlight. Mildew, typically black or white in colour, can grow on a variety of surfaces, including fabric cushions. Mildew will stain the cushions and give off an unpleasant musty odour. But if you act quickly, you can remove the mildew from a fabric cushion before the stain sets in.
Remove the fabric cushions from the furniture. Take the cushions outside and dust off as much mildew as possible from the surface.
Mix 1/2 cup of borax with 2 cups of hot water until the borax is thoroughly dissolved into the water. Alternatively, mix equal parts water and rubbing alcohol.
Saturate a sponge in the mixture. Scrub the mildew on the fabric cushions until it is no longer visible.
Dampen a clean cloth in cool water. Rinse the fabric cushion by wiping with the cloth.
Blot the cushion with a dry towel to absorb excess liquid. Allow the fabric cushion to air-dry out of direct sunlight.
Test the mildew removal method on an inconspicuous area of the fabric cushion. If damage or discolouration occurs, discontinue use. An alternative cleaning method---if the fabric cover can be removed from the cushion---is to launder it as you normally would, except add 1 cup of white vinegar to the wash.