Rust

in metallurgy, a rough spot on the surface of a metal. Iron rust is typical. It is a reddish or orange-yellow stain due to a combination of the metal and oxygen derived from moist air. Rusting has been called a slow combustion or burning. In botany rust is a decayed spot due to the growth of a small plant--a parasitic fungus--that eats away the plant on which it lives. Rusted oats, for example, are oats whose stalks have been badly eaten by microscopic plants whose colonies have lodged in spots on the leaves and stems of the oats. Rusts spread by means of dust-like spores and are of many kinds, one or more for each of a large number of fruits and field crops and vegetables. They usually grow unnoticed until they are ready to cast off spores when they break out in brown blotches or spots. Crowded plants and moist weather are likely to develop rusty grain. See Fungus