Wooden fences often turn a slight green and black colour when mildew and algae begin to form. Since wood is extremely porous, moisture often sticks around long enough for this nasty stuff to start to grow. If left untreated, it can rapidly spread and become not only an eyesore, but also a bit dangerous to your health if you come in close contact with the mildew and algae on a regular basis.
Rent or purchase a power washer that operates at a 15000 or 2000 psi (not 3000 or 3500 psi, which will be too strong). Power washing will clean away all mildew and algae from your fence much faster than scrubbing by hand.
Test an area of the fence that is not seen or touched by people since power washing can damage the surface of the wood if it is done incorrectly. Try different tips and distances to the fence to be sure you are not damaging the wood. Start about 45 cm (18 inches) away and use a 25-degree tip.
Tie back any plants and bushes. Clear any other obstructions so you have a clear path to clean your fence.
Fill up your pump and hose with water before turning on the power washer. You do this by turning on the hose spigot first while holding the wand button. Once water starts to squirt out, start the pump. However, you should follow all of the manufacturer's directions when using your pump.
Power wash your fence with a 25-degree tip. Move the power washer up and down the fence panels about 45 cm (18 inches) away. You can nudge forward to get more mildew and algae off, but be sure you don't damage the wood. The wood will start to change colour and become the bright fence you originally purchased. Move on to the next panel once no more colour changing occurs. As you get more comfortable with the power washer, you can try to switch to a 15-degree tip, which will work more quickly, but can cause more damage if it is not used carefully.
Repair any damaged panels and countersink any loose nails. Then you are ready to stain and seal your fence preserving its quality.