An oidium (plural: oidia) is an asexually produced

fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

spore that (in contrast to conidia) is presumed not to constitute the main reproductive preoccupation of the fungus at that time. The hypha breaks up into component cells/ small pieces and develop into spores. Oidia cannot survive in unfavourable conditions.

References

* Illinois Mycological Association glossar

Fungal morphology and anatomy {{Mycology-stub pt:Oídio