The mold comes from the oak...usually if your bed is oak. Sometimes it may be ash as well. You can spray on a fungicide/bleach combination similar to antifouling compound from the bottom of boats but be very careful. The mold is bad, but so is the cure.

you have to watch bleach and water on an oak bed because the combination is acidic and it will turn the oak jet black where it contacts any iron...the bolts or ribs etc.

i usually mix the anti fungicide stuff in a mix of wd40 and brush it into the spaces...it penetrates and preserves and cures.

To restore an old truck is the work of a man...to restore a man is the work of God.