OIDIUM

noun (plural oidia) Etymology: New Latin, from o- + -idium Date: 1857 1. 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, one of the small conidia borne in chains by various fungi (as an ~), a powdery mildew especially of grapes caused by an ~