yellow spot vs plant disease

yellow spot and plant disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
yellow spot	Yes	No	No	No
plant disease	Yes	No	No	No

As nouns, plant disease is a hypernym of yellow spot; that is, plant disease is a word with a broader meaning than yellow spot:

yellow spot: any of several fungous or viral diseases characterized by yellow spotting on the leaves
plant disease: a disease that affects plants

yellow spot (noun)	plant disease (noun)
a small yellowish central area of the retina that is rich in cones and that mediates clear detailed vision	a disease that affects plants
any of several fungous or viral diseases characterized by yellow spotting on the leaves