Try a solution of vinegar and water in a spray bottle. Let it soak in the area before scrubbing with a firm bristled brush. Hose off and enjoy!

Bleach and a scrub brush. If that doesn't work try acid.

They make all kinds of deck wash products tha would work great for you.

A power washing works well.

you need to powerwash them with a strong solution that kills all types of mold. if you would like someone to take care of it for you call All Seasons Cleaning 302-733-7610