Prepare the wood deck treatment solution by mixing 3 quarts of water, 1 quart of oxygen bleach, and 1/4 cup of ammonia-free liquid dishwasher detergent. If you don't have bleach, you can substitute vinegar.
Homemade Deck Cleaner for Outdoor Cleaning. If your deck has visible mildew stains, add 1 cup of powdered borax to the solution at the same time you add .
Update for 2018: How to Remove Mold and Algae From Decks. Deckstainhelp.
Use a non-chlorine laundry bleach to remove the stain. (This works especially well if the stain is from mold, mildew or algae.) Apply it to the affected area, then scrub with a nylon brush. Rinse the area with water.
Next time it's time to clean your deck, try a homespun concoction that will. in the weather, you may encounter issues with mildew and/or algae on your deck.