oïdium

Century Dictionary and Cyclopedia

1. n. A genus of parasitic fungi, having the sterile hyphæ decumbent and the sporophores erect. The conidia are ovoid, rather large, and hyaline or pale. They are thought to represent the conidial stages of various Erysipheæ. O. Tuckeri, the European grape-mildew, which produces only conidia, was thought to be the same as the destructive American grape-mildew, but the latter is now known to produce oöspores, and is referred to Peronospora viticola. Thirty-five species of Oidium are admitted by Saccardo. See Peronospora, grape-mildew, grape-rot, mildew, Erysipheæ.
2. n. In botany, one of a considerable chain of conidia.

GNU Webster's 1913

1. n. (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.