Mildew is a common problem on wood fences, but although mildew may live on the wood, it does not feed on it, according to Jeff Beneke in "The Fence Bible: How to Plan, Install and Build Fences and Gates to Meet Every Home Style and Property Need, No Matter What Size Your Yard." One of the best ways to remove the discolouration is to wash the wood fence with a bleach solution. The bleach will kill off the fungus and brighten the wood.
Pour 1 qt. of bleach and 2 qt. of warm water into a bucket. Add ¼ cup of dish detergent, and mix the solution with a wooden spoon.
Pour the solution into a garden sprayer, and spray the wood. If a garden sprayer is unavailable, dip a brush with stiff bristles into the solution and scrub the fence.
Wash the solution off the fence with water from a garden hose. It is important to rinse the fence within 15 minutes so the grime does not dry on the wood. Let the fence dry. In "The Fence Bible: How to Plan, Install, and Build Fences and Gates to Meet Every Home Style and Property Need, No Matter What Size Your Yard," Jeff Beneke suggests coating the fence with mildewcide to protect it from further mildew build-up.