disease caused by a certain group of fungi and characterized by reddish brown spots on the foliage and/or the formation of stem galls.

Merriam-Webster Online Dictionary
rust (noun)
1.
a) the reddish brittle coating formed on iron especially when chemically attacked by moist air and composed essentially of hydrated ferric oxide
b) a comparable coating produced on a metal other than iron by corrosion
c) something resembling rust - accretion
2.
corrosive or injurious influence or effect
3.
any of numerous destructive diseases of plants produced by fungi (order Uredinales) and characterized by usually reddish-brown pustular lesions , also a fungus causing this
4.
a strong reddish brown
rust (verb)
intransitive verb
1.
to form rust become oxidized - iron rusts
2.
to degenerate especially from inaction, lack of use, or passage of time - most men would … have allowed their faculties to rust T. B. Macaulay
3.
to become reddish brown as if with rust - the leaves slowly rusted
4.
transitive verb
to be affected with a rust fungus
1.
to cause (a metal) to form rust - keep up your bright swords, for the dew will rust them Shakespeare
2.
to impair or corrode by or as if by time, inactivity, or deleterious use
3.
to cause to become reddish brown turn the color of rust
rust (Wikipedia)

This article is about the chemical compound. For other uses, see Rust (disambiguation).
Colors and porous surface texture of rust

Rust is an iron oxide, a usually red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Several forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).

Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. Rusting is the common term for corrosion of iron and its alloys, such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called rust.

Other forms of rust exist, like the result of reactions between iron and chloride in an environment deprived of oxygen. Rebar used in underwater concrete pillars, which generates green rust, is an example. Although rusting is generally a negative aspect of iron, a particular form of rusting, known as "stable rust," causes the object to have a thin coating of rust over the top, and if kept in low relative humidity, makes the "stable" layer protective to the iron below, but not to the extent of other oxides, such as aluminum.

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