Mold in the Dungeon

Mold.  

The only threatening mold we players deal with tends to be what grows in the kitchen and always-wet walls. 

PCs in DnD (and other rpgs), on the other hand, have multiple molds to contend with.

Stock photo of yellow mold

If I had to guess, the inclusion of molds as hazards in DnD stems from one specific book.  Those aware of the venerable Appendix N: Inspirational and Educational Reading have likely read Margaret St. Clair's 'Sign of the Labrys.'  For those who haven't (or have fading memories - it was published in 1963), the key takeaways from it are that it is basically a megadungeon and that there is a prevalence of molds and fungal growths that do bad things to people, notably wiping out 9/10s of the Earth.  It is a fun read.

(So are several books on that list.  Others not so much, either due to not aging well or simply being terrible books.  Appendix N Bookclub addresses them all, or will eventually.  A great podcast. (Sign of the Labrys, Shadow People)).

Reading lists aside, molds and slimes and fungi as threats have been in our game since its inception, or at least since Moldvay Basic, the earliest ruleset I own.  And 1981 is pretty close in time to OD&D.

Sometimes they have been found alongside traditional monsters, with Yellow Mold between Wolf and Zombie in B/X and in the middle of the M's in AD&D.  After all, they are technically alive and will kill a PC.

Other times, these trap-monsters can be found perhaps more appropriately under their own heading, typically something like Hazards (5e) or Dungeon Hazards (maybe 3x - my books are long gone).  

Which makes sense, because they are hazards.  Unfortunately, since they are not in the newer monster manuals, but in the DMGs, such things don't appear nearly as often in adventures as they should. Molds (brown, russet, yellow) all have their purpose, which is to make treasure recovery more difficult while presenting different challenges than 'hit it with sword or spell.' 

Brown Mold feeds and grows on warmth, and makes recovering a treasure that it has grown on a challenge, given that warm-blooded beings give off enough heat to make it grow.  Fireball it from a distance and see what happens.  I dare you.  

Brown Mold also turns the 'kill it with fire' paradigm on its head, a method that works for other molds.  

Russet Mold turns humanoids into Vegepygmies and quadrupeds into Thornies.  Permanently, as S3 Expedition to the Barrier Peaks teaches us.  I imagine a properly-worded Wish might bring a PC back to normal, but little else.  At least the PC's gear survives to be picked up by the next adventuring party that passes through.

Erol Otus, from S3 Expedition to the Barrier Peaks, 1980

Yellow Mold is the classic threat mold, found on plates to make them appear golden, growing on skeletons to make melee with them a danger again, and on sacks of grain in abandoned storerooms because it kinda makes sense for it to be there.  Yellow mold basically exists to punish careless PCs (and players as PCs succumb and die) for doing what they do - looting, fighting, and searching.  

I like Yellow Mold because it grows on anything, so can and will be found on anything nonliving.  Adding it to constructs and undead is a nasty trick (ooh... a Yellow Mold breath weapon?), that can be telegraphed visually (another strike against colorblind darkvision) if the DM is kind or the PCs ask.

Mostly, though, Yellow Mold punishes folks for not carrying lit torches.  Between Yellow Mold and Green Slime (and maybe spider webs and Rot Grubs), PCs (especially in 5e) heeding the clarion call of "I have darkvision" are at an extra disadvantage because not having a ready fire source means time lost and that means PCs die.  Decisions, decisions...

Now, Molds are a type of plant life, which means some druids can communicate with and otherwise affect them.  What this looks like in game, I don't know, but am excited to find out.

I write all this because, yes, my megadungeon features all these molds, and more besides.  For those needing/wanting more inspiration (and a nifty table) in this regard, d4 Caltrops has it covered (pun intended).  Built By Gods Long Forgotten has a remix of slimes and molds, as well (among other critters), as does this thread at EnWorld.