It's been a damp and rainy Spring thus far, which has led to a bumper crop of mushrooms sprouting all over the neighborhood, as my son as been keen to point out. Seeing them, I found myself remembering the first D&D module I ever owned, In Search of the Unknown, whose original and revised covers both depict adventures exploring a garden of giant fungi.
the cover of the AD&D Players Handbook, I can't deny that a close second is the cover of module B1.
An interesting side effect of this association in my imagination is that I also strongly associate mushrooms and other fungi with D&D, an association made all the stronger because of their regular presence in a lot of old school artwork. And of course the game itself includes so many mycoid monsters, the shrieker, violet fungi, and yellow mold being the three most iconic, never mind all the new ones introduced by Gygax as he worked up The Temple of Elemental Evil for publication.
I'm sure that, in reality, old school D&D didn't include nearly as much fungal imagery as I imagine, but the fact that it seems to have done so is, I think, significant. Mushrooms speak to me of an older literary conception of fantasy, harkening back to 19th century tales like Alice's Adventures in Wonderland and Journey to the Center of the Earth. That's no surprise, of course, considering the influence such stories had on Gygax, Arneson, and other early designers. So, when I see mushrooms in contemporary fantasy art, as I do in Steve Zieser's Labyrinth Lord illustrations, it's like a code that tells me these people share the same conception of D&D that I do.
With that, I leave you with a handful of images of mushrooms and fungi from old school D&D modules, particularly from the fungus-filled In the Dungeons of the Slave Lords.