I use Sodium Hypochlorite (liquid pool bleach) 1/2 gallon tor 1 1/2 gallon of water in a pump up garden sprayer. For heavier stain increase the pool bleach formula. (Warning: this will strip off the existing wood stain if applied directly)
I've also used oxalic acid crystals 1 cup crystals to 2 gallons of water. Same garden sprayer application.
Gently powerwash off.
Hope this helps.
Rick