The Collaborative International Dictionary of English v.0.48:

Oidium \O*["i]d"i*um\, n. [NL., dim. fr. Gr. w,'o`n egg.] (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 (O["i]dium
   Tuckeri), which has caused much injury to grapes.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arthrospore \Ar"thro*spore\, n. [Gr. ? joint + E. spore.]
   (Bacteriol.)
   1. A bacterial resting cell, -- formerly considered a spore,
      but now known to occur even in endosporous bacteria.
      [Webster 1913 Suppl.]

   2. one of the small conidia occurring arranged in chains in
      certain fungi. Called also oidium.
      [PJC] -- Ar`thro*spor"ic, Ar*thros"po*rous, a.
      [Webster 1913 Suppl.]