oïdium

from The Century Dictionary.

• noun A genus of parasitic fungi, having the sterile hyphæ decumbent and the sporophores erect.
• noun In botany, one of a considerable chain of conidia.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A genus of minute fungi which form a floccose mass of filaments on decaying fruit, etc. Many forms once referred to this genus are now believed to be temporary conditions of fungi of other genera, among them the vine mildew (Oïdium Tuckeri), which has caused much injury to grapes.