head smut vs smut fungus

head smut and smut fungus both are nouns.

Word	Noun	Adjective	Verb	Adverb
head smut	Yes	No	No	No
smut fungus	Yes	No	No	No

As nouns, smut fungus is a hypernym of head smut; that is, smut fungus is a word with a broader meaning than head smut:

head smut: smut fungus attacking heads of corn or sorghum and causing a covered smut
smut fungus: any fungus of the order Ustilaginales

Other hypernyms of head smut include smut.

head smut (noun)	smut fungus (noun)
smut fungus attacking heads of corn or sorghum and causing a covered smut	any fungus of the order Ustilaginales