jùn is the cooking-and-foraging reading for edible mushrooms, strongest in southwestern China where 野生菌 (wild mushrooms) are a regional delicacy. It appears in named species like 牛肝菌 (porcini) and 马勃菌 (puffball). For germs and laboratory fungi the same character is read jūn instead.
Top grass radical, the indexing semantic. Classical taxonomy filed fungi as low plants, so 菌 sits in the botanical family with 花, 草, and 药. The radical covers both the microbe sense (jūn) and the mushroom sense (jùn).
Bottom 囷 supplies the sound, qūn shifting to jūn through a regular softening of the velar initial. It shows 禾 (grain) inside 囗 (enclosure), a round granary where grain was stored, a faint echo of damp things sprouting in storage. The easiest hook: the qūn shape supplies the jūn sound.
