Cobweb is a fungal disease caused by a number of closely related species of Cladobotryum. In North America, no highly pesticide resistant varieties have been identified to date although resistance seems to be increasing.
Disease symptoms can vary, usually dependant on the time of infection. The most commonly observed symptom of Verticillium (or dry bubble disease) is brown spotting on the mushroom caps.
This fungal disease is caused by the fungus Mycogyne perniciosa and is common although often not serious. When it develops early in a crop and if not controlled, it can cause considerable crop loss.
Trichoderma harzianum mycelium begins as a gray color and then changes to white becoming very dense. After fruiting, its spores turn it a dark green. There are many other types of molds that also are green associated with mushroom compost.
Pseudomonas tolaasii, the cause of bacterial blotch is an aerobic, non-spore forming fluorescent bacterium. Pseudomonads are common bacterium easily isolated from field soil, compost and casing.
Mushrooms are grown in specially prepared compost which favors the growth of mushroom mycelium. In the same way a plant growing out of place is referred to as a weed, unwanted mold (also referred to as fungi), growing out of place are often referred to as weed molds.