Rusts are plant diseases caused by
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
ic
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
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
''Puccinia'' is a genus of fungi. All species in this genus are obligate plant pathogens and are known as rusts. The genus contains about 4000 species.
The genus name of ''Puccinia'' is in honour of Tommaso Puccini (died 1735), who was an Itali ...
'', 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
An aecium (plural aecia) is a specialised reproductive structure found in some plant pathogenic rust fungi that produce aeciospores. Aecia may also be referred to as "cluster cups". The term aecidium (plural aecidia) is used interchangeably but i ...
,