Bark Frameworks

Environment - Biological Agents

These stains are sometimes called foxing. Causes are uncertain, but probably fungal growth, encouraged by high humidity, is a main factor.

Mold spores are always in the air.  In order to grow, however, they need specific conditions.  They like it warm (above 75 degrees °F), moist (above 60% RH), and an area that is dark, with little air circulation.  A basement is a good nursery for mold - or the area behind a frame on a damp wall.

Insects, of course, are drawn to all sorts of organic materials.