Might well depend on the species as to which conditions are most favorable, but AIUI most of your farmed mushrooms are grown on other-than-poop substrates: grai

Might well depend on the species as to which conditions are most favorable, but AIUI most of your farmed mushrooms are grown on other-than-poop substrates: grain, straw, wood and wood pellets, coffee grounds, shredded coconut husk. Probably still a good idea to clean them off, though: none of those other things are particularly tasty :)