o·id·i·um 
(ō-ĭdē-əm)
(ō-ĭdē-əm)n. pl. o·id·i·a (-ē-ə)
1. A thin-walled spore produced asexually by fragmentation in certain filamentous fungi.
2. Any of various fungi that cause powdery mildew.
[New Latin Ōidium, fungus genus, from Greek ōion, egg; see OO-.]