I have been having the same problem. The rains have arrived in the PNW, so I built a makeshift cover to keep them dry, but still had new mold growth just do to the high humidity. I have trimmed away all existing mold and now have been using a product that contains Bacillus subtilis. It is a bacteria that out-competes the bad stuff. Smells a little like cider, but seems to be working so far!