Spoiler warning: Plot and/or ending details follow.
Fungus (plural: fungi) were a type of eukaryotic organism that included mushrooms, yeasts, and mold. Earth based fungi were noted for sharing a large amount of DNA with humans.
On Tarsus IV, seeds imported from Epsilon Sorona II interacted poorly with the soil, spawning a fungus that destroyed most of the planet's food supply. In the years following the crisis a great deal of work went in to prevent similar fungal infections from occurring elsewhere. (DSC novel: Drastic Measures).
Abaris natives had a fungal-based physiology. (TOS comic: "Aberration on Abaris")
On Direidi, a type of fungus is used to distill sparkling wines. (TOS novel: How Much for Just the Planet?)
Fungus could be used in the manufacturing of Ketracel-white. (DS9 episode: "Statistical Probabilities")
Doctor Mora Pol once suggested exposing an infant Changeling to fungi in order to train it to assume different shapes. (DS9 episode: "The Begotten")