Source : Webster's Revised Unabridged Dictionary (1913)

Rust \Rust\, n. [AS. rust; akin to D. roest, G. & Sw. rost,
   Icel. ry[eth]; -- named from its color, and akin to E. red.
   [root]113. See {Red}.]
   1. (Chem.) The reddish yellow coating formed on iron when
      exposed to moist air, consisting of ferric oxide or
      hydroxide; hence, by extension, any metallic film of
      corrosion.

   2. (Bot.) A minute mold or fungus forming reddish or rusty
      spots on the leaves and stems of cereal and other grasses
      ({Trichobasis Rubigo-vera}), now usually believed to be a
      form or condition of the corn mildew ({Puccinia
      graminis}). As rust, it has solitary reddish spores; as
      corn mildew, the spores are double and blackish.

   Note: Rust is also applied to many other minute fungi which
         infest vegetation, such as the species of {Ustilago},
         {Uredo}, and {Lecythea}.

   3. That which resembles rust in appearance or effects.
      Specifically: (a) A composition used in making a rust
      joint. See {Rust joint}, below.
      (b) Foul matter arising from degeneration; as, rust on
          salted meat.
      (c) Corrosive or injurious accretion or influence.

                Sacred truths cleared from all rust and dross of
                human mixtures.                   --Eikon
                                                  Basilike.

   Note: Rust is used in the formation of compounds of obvious
         meaning; as, rust-colored, rust-consumed, rust-eaten,
         and the like.

   {Rust joint}, a joint made between surfaces of iron by
      filling the space between them with a wet mixture of
      cast-iron borings, sal ammoniac, and sulphur, which by
      oxidation becomes hard, and impervious to steam, water,
      etc.

   {Rust mite} (Zo["o]l.), a minute mite ({Phytopius oleivorus})
      which, by puncturing the rind, causes the rust-colored
      patches on oranges.

Source : WordNet®

rust mite
     n : any of several varieties of mite that burrow into plants and
         cause a reddish-brown discoloration on the leaves or
         fruit