mildew vs downy mildew

mildew and downy mildew both are nouns.

Word	Noun	Adjective	Verb	Adverb
mildew	Yes	No	Yes	No
downy mildew	Yes	No	No	No

As nouns, downy mildew is a hyponym of mildew; that is, downy mildew is a word with a more specific, narrower meaning than mildew:

mildew: a fungus that produces a superficial (usually white) growth on organic matter
downy mildew: any of various fungi of the family Peronosporaceae parasitic on e.g. grapes and potatoes and melons

Other hyponyms of mildew include powdery mildew, false mildew.

mildew (noun)	downy mildew (noun)
a fungus that produces a superficial (usually white) growth on organic matter	any of various fungi of the family Peronosporaceae parasitic on e.g. grapes and potatoes and melons
the process of becoming mildewed

mildew (verb)	downy mildew (verb)
become moldy; spoil due to humidity