Oidium n. : 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 (Oidium Tuckeri), which has caused much injury to grapes.

Example:
Can the powdery mildew Oidium lycopersicum be stored in the freezer in order to inoculate tomato plants?

Taxonomy
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Defination from the web:
Oidium (genus). For the fungus that causes powdery mildew on grapes, see Uncinula necator. An oidium (plural: oidia) is an asexually produced fungal spore that (in contrast to conidia) is presumed not to constitute the main reproductive ...