Rusts are plant diseases caused by
pathogenic
fungi of the order Pucciniales (previously known as Uredinales).
An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''
Puccinia'', are currently accepted.
Rust fungi are highly specialized plant pathogens with several unique features. Taken as a group, rust fungi are diverse and affect many kinds of plants. However, each species has a very narrow range of hosts and cannot be transmitted to non-host plants. In addition, most rust fungi cannot be grown easily in pure culture.
A single species of rust fungi may be able to infect two different plant hosts in different stages of its life cycle, and may produce up to five
morphologically and
cytologically distinct spore-producing structures viz.,
spermogonia,
aecia,