If I don’t use fresh store-bought mushrooms fast enough, they go weird and slimy. It seems ironic that the fruiting body of a fungus (which lives to rot stuff) is eventually rotted. My question is: Does the same fungus that grew the mushroom rot itself, or is it another species of fungus that does the deed?
Probably the fungal equivalent of Dean Swift’s memorable light verse:
sex, drugs, and rock n’ roll…
When mushrooms go bad, what rots them?
The same organisms that rot everything else, fungi, viruses, molds, microbes, small anamacules etc.
Intresting question though…
I’ve eaten slimy and not-so-slimy mushrooms and still exist. Never tried the other mushroom thing. Yet. 
Did have a few anamacules last night. They were on the menu.
There’s probably a good bit of good ol’ fashioned oxidation going on, too, which doesn’t require any outside microbes.
Bacteria, most likely.