onion smut vs smut fungus

onion smut and smut fungus both are nouns.

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

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

onion smut: smut fungus causing blackish blisters on scales and leaves of onions; especially destructive to seedlings
smut fungus: any fungus of the order Ustilaginales

Other hypernyms of onion smut include smut.

onion smut (noun)	smut fungus (noun)
smut fungus causing blackish blisters on scales and leaves of onions; especially destructive to seedlings	any fungus of the order Ustilaginales