Puccinia graminis

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. [1913 Webster]

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. [1913 Webster]

Note: Rust is also applied to many other minute fungi which infest vegetation, such as the species of {Ustilago}, {Uredo}, and {Lecythea}. [1913 Webster]

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. [1913 Webster]

Sacred truths cleared from all rust and dross of human mixtures. --Eikon Basilike. [1913 Webster]

Note: Rust is used in the formation of compounds of obvious meaning; as, rust-colored, rust-consumed, rust-eaten, and the like. [1913 Webster]

{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. [1913 Webster]