ōˈidēəm noun
Etymology: New Latin, from o- + -idium
1.
a. capitalized : a genus of imperfect fungi (family Moniliaceae) including many now considered to represent conidial stages of various powdery mildews — compare blastomycetes , thrush
b. plural oid·ia -ēə : a fungus of this genus
c. plural oidia : one of the small conidia borne in chains by various fungi (as members of the genus Oidium ) — called also arthrospore
2. plural oidia : a powdery mildew caused by a fungus of the genus Oidium especially in the grape — compare oidiomycosis