Thanks for all the feedback, everybody! I understand about perched water tables, and that's definitely something to watch out for. But as I've been growing bulbs for fifteen years and never had this problem before, it's difficult to figure out why it would start now. I didn't change anything. Maybe some fungus spores just happen to have blown in, but for now I think my prime suspect is the rotting wood. We'll see. If the fiberglass sheets solve the problem, I think I'll have my proof. Thanks, Mike San Jose, CA