flax rust vs rust fungus

flax rust and rust fungus both are nouns.

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

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

Other hypernyms of flax rust include rust.

flax rust (noun)	rust fungus (noun)
fungus causing flax rust	any of various fungi causing rust disease in plants