Verticillium dry bubble (Verticillium fungicola (Preuss) Hassebrauk var. funigicola), Green mold (Trichoderma spp.), Cottonweb mold (Hypomyces sp.) and Bacterial blotch (Pseudomonas fluorescens biotype G) are the more common problems in mushroom cultivation that come to mind. Other problems can be caused by nematodes, other fungi and viruses. Here are some helpful links: