Old School Essentials has the Incandescent Grottoes which includes “Adventurers who delve within may discover odd mosses and fungi, a ruined temple complex, and the lair of a crystal-eating dream dragon.” Among other things. I remember part of Winter’d Daughter with a bag of various fungi that had all kinds of weird effects when eaten (it was in the land of Faerie, of course), and Dolmenwood has quite a bit about Fungi included in it, mostly for their medicinal and other odd properties when foraged for.