oidium

noun2

1. •
   A fragile spore produced by some fungi.

   • However, in some forms mainly some Hymenomycetes it occurs by the formation of oidia. The oidia are of two main types. — 2004
2. •
   The fungus Erysiphe necator (= Uncinula necator), which produces powdery mildew in grapes.