Mix one part ammonia with three parts hot water in a spray bottle. Put on rubber gloves. Spray the mixture directly on wooden fencing. Scrub with a scrub brush to get into the wood grain and crevices. Ammonia effectively kills organisms, and its vapors will not harm you in an outdoor environment. Spray off the ammonia with a garden hose. This method can also be used substituting bleach or chlorine. Just don't combine the two, because it will react and release a toxic gas.
Vinegar and Hot Water
Mix one part white vinegar with two parts hot water in a spray bottle for use on vinyl or metal pickets. Let the vinegar sit on the pickets for a minute or two. Vinegar will raise the pH of the surface, killing the algae. Wipe it off with a clean, soft cloth to avoid scratching the vinyl or painted metal. Do not rinse, but leave vinegar residue on the fence.
Hydrogen Peroxide Cleaner
Mix one part hydrogen peroxide with two parts hot water in a spray bottle. Spray the pickets on wood or metal, and let it sit for an hour. The hydrogen peroxide will oxidize the algae, killing it. If wood pickets are paint or weather-damaged, scrub the solution in with a scrub brush. Rinse off with a hard stream of water from a garden hose, and wipe dry. This solution, as well as the other two, can clean mold and mildew in your kitchen and bathroom, too.