Looks to me like you got a leak there. You might try something for cleaning and treating a deck
^^^ This. Try oxalic acid for cleaning, or a pressure washer soap that has a mildewcide.
Can that be used for deck railing that is painted?^^^ This. Try oxalic acid for cleaning, or a pressure washer soap that has a mildewcide.
Can that be used for deck railing that is painted?
Can that be used for deck railing that is painted?