A mushroom was a fungus. The Great Power was an Earth mushroom that could bestow superhuman powers. (PROSE: Doctor Who and the Daleks)
The First Doctor, Vicki, Steven Taylor and Robert Zierath visited a planet with giant talking mushrooms. (PROSE: The Schoolboy's Story)
The Doctor's TARDIS assumed the form of a giant mushroom when it arrived on an alien planet. (AUDIO: The Beginning)