Mold and mildew can be dangerous to people and damaging to wood, so it's always a good idea to clean it off when it comes up. While you're at it, applying a fresh coat of sealant can prevent the problem from popping back up.
Step 1 - Preparation
If you're removing mold and mildew from a deck or wooden steps, remove all of the patio furniture and sweep the area first. Cover any bushes, plants or trees that are nearby—you don't want the cleaning solution to get on them. Put all pets inside.
If you're removing mold and mildew from any other exterior surface such as wood siding, wood patio furniture or fences, hose down the wood and sweep it clean with a broom. Cover plants, flowers and trees in these areas also to prevent spraying with the mildew remover.
Step 2 - Removal of Mold and Mildew
Mix the deck cleaner, ideally one specifically designed to be a mold and mildew remover, according to the manufacturer's instructions on the container. Pour the mixture in your pressure washer, plug it into the garden hose, and connect it to a power source that is grounded.
If power levels are available on your power washer, set it at a medium level to start. Use a sweeping motion and saturate the wood with the cleaner. Generally you will see most of the mold and mildew wash away with this step. On stubborn areas, you may want to turn the power setting up a bit and go over it again.
If any areas are still stubborn, pour a bit of the concentrated deck wash onto the area, let it sit, scrub with an old broom, then rinse thoroughly to remove it.
Disconnect your garden hose once all the mold and mildew are removed and use it to rinse the wood completely with clean water. If you think any of the deck wash has gotten on plants, wash them thoroughly and also the soil around them. Most deck and wood cleaners have a high concentration of chlorine, which is harmful to plants and lawns.
Step 3 - Prevention
Allow the wood to dry for several days. During hot dry weather this will happen in just two or three days. If it happens to rain, wait until it's totally dry again. You don't want to seal in any moisture. Wood will lighten in color as it becomes more dry.
Coat the wood with a good wood sealer/water repellent for exterior use. This will cause water to bead up and run off the surface instead of penetrating and allowing mold and mildew to grow. Be especially careful to give it a good coating in areas that are not in sunlight as mold and mildew grow more easily in these areas. If the wood has never been coated, you may want to apply a second coat to be sure it's sealed well.