Paul Stamets was an astromycologist. After seeing his presumed dead partner, Hugh Culber, in the mycelial network he studied, he started wondering if it could be more than a mental fabrication, because astromycology has taught him that nothing is ever truly gone. (DIS: "New Eden")
Mycology is the study of fungi, while astro- (like exo- and xeno-) is often used both in real life and in Star Trek to indicate a the part of a field looking at extraterrestrial examples. For example, astromycology is presumably a subfield of astrobiology.