Definition of oidium
1a
: any of a genus (Oidium of the family Moniliaceae) of imperfect fungi many of which are now considered to be conidial stages of various powdery mildews
b
: one of the small conidia borne in chains by various fungi (such as an oidium)
— called also arthrospore
2
: a powdery mildew especially of grapes caused by an oidium
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Merriam-Webster unabridged
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Time Traveler for oidium
The first known use of oidium was in 1888
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