Read jūn for the microbiology sense: 细菌 (bacteria), 真菌 (fungus), 杀菌 (to sterilize), 病菌 (germ). When 菌 names an edible mushroom it switches to jùn, as in 牛肝菌 (porcini) and 野生菌 (wild mushrooms).
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.
