An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus Puccinia, are currently accepted.

Most species of rust fungi are 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, uredinia, telia, and basidia in successive stages of reproduction.

Rust fungi grow intracellularly, and make spore-producing fruiting bodies within or, more often, on the surfaces of affected plant parts.

Rusts get their name because they are most commonly observed as deposits of powdery rust-coloured or brown spores on plant surfaces.

Rust fungi are major concerns and limiting factors for successful cultivation of agricultural and forest crops.

[6] All rusts are obligate or biotrophic parasites, meaning that they require a living host to complete their life cycle.

[7] Cereal crops can be devastated in one season; oak trees infected in the main stem within their first five years by the rust Cronartium quercuum often die.

This can be contrasted with an autoecious fungus, such as Puccinia porri, which can complete all parts of its life cycle on a single host species.

Some genera of rust fungi, especially Puccinia and Uromyces, comprise species that are capable of parasitizing plants of many families.

Rust fungi decrease photosynthesis and elicit the emissions of different stress volatiles with increasing severity of infection.

A rust spore typically germinates on a plant surface, growing a short hypha called a germ tube.

This involves orienting to ridges created by epidermal cells on the leaf surface, and growing directionally until it encounters a stoma.

[15] It is thought that the whole process is mediated by stretch-sensitive calcium ion channels located in the tip of the hypha, which produce electric currents and alter gene expression, inducing appressorium formation.

[21] Elvin C. Stakman initiated the scientific study of host resistance, which had heretofore been poorly understood and handled by individual growers as part of the breeding process.

[citation needed] Puccinia graminis is a macrocyclic heteroecious fungus that causes wheat stem rust disease.

The durable spore type produced on the alternate host allows the disease to persist in wheat even in more inhospitable environments.

[citation needed] Although the disease cannot be stopped by removal of the alternate host, the life cycle is disrupted and the rate of evolution is decreased because of reduced genetic recombination.

Cedar-apple rust disease, for example, can persist despite removal of one of the hosts since spores can be disseminated from long distances.

Southern rust's distinguishing characteristic is that pustules form mostly on the upper leaf surface and spores are more orange in color.

Southern rust spreads more quickly and has a higher economic impact when hot, humid weather conditions persist.