Damp wood is a place that mold and mildew thrive. If you want to stop this from occurring on your deck, you must first kill and remove the mold and mildew, then treat your wood deck with a sealant that has a fungicide added to it. The hard part is killing and removing the mold and mildew, and knowing what products to use to do so without harming the wood.
Pressure washing will power away the mold and mildew along with any dirt and algae that may be on your deck.
Bleach can be used alone or with the power washer to kill mold and mildew. Bleach will kill spores deep in the woods fibers.
Vinegar works similarly to bleach but without the chemical composition.
Laundry Detergent with Bleach Additive
Scrubbing your deck with a laundry detergent such as Tide With Bleach will remove and kill mold and mildew. You will need to rinse thoroughly to get all the residue off of the wood.
A good hot day is the ideal time to clean the deck, and letting it dry in the hot sun is an easy way to kill mold spores. Dry wood is unattractive to the mold and mildew, and heat will kill the spores.