The Collaborative International Dictionary of English v.0.48:

Mildew \Mil"dew\, n. [AS. melede['a]w; akin to OHG. militou, G.
   mehlthau, mehltau; prob. orig. meaning, honeydew; cf. Goth.
   milip honey. See Mellifluous, and Dew.] (Bot.)
   A growth of minute powdery or webby fungi, whitish or of
   different colors, found on various diseased or decaying
   substances.
   [1913 Webster]

   powdery mildew a fungal disease of plants caused by an
      ascomycete of the order Erysiphales, characterized by a
      powdery white film on the surface of the affected plants.
      It is damaging to, e.g., roses and lilacs. Also, a fungus
      that causes such a disease.
      [PJC]

WordNet (r) 3.0 (2006):

powdery mildew
    n 1: any of various fungi of the genus Erysiphe producing
         powdery conidia on the host surface