My problem was the cold. The closet in my sewing room is an outside wall. If I put anything against the wall in winter, heat cannot get behind it, and frost forms there. Then when it gets warmer, the frost melts and turns to mold. I have learned to put a small board for a spacer at the back of shelves to keep things away from those back walls. I got rid of the mold with a little bleach in the wash water and in the water used for wiping down the shelves.