rust fungus vs fungus

rust fungus and fungus both are nouns.

Word	Noun	Adjective	Verb	Adverb
rust fungus	Yes	No	No	No
fungus	Yes	No	No	No

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

rust fungus: any of various fungi causing rust disease in plants
fungus: an organism of the kingdom Fungi lacking chlorophyll and feeding on organic matter; ranging from unicellular or multicellular organisms to spore-bearing syncytia

rust fungus (noun)	fungus (noun)
any of various fungi causing rust disease in plants	an organism of the kingdom Fungi lacking chlorophyll and feeding on organic matter; ranging from unicellular or multicellular organisms to spore-bearing syncytia