Uredinales

(mycology) An order of parasitic fungi of the subclass Heterobasidiomycetidae characterized by the teleutospore, a spore with one or more cells, each of which is a modified hypobasidium; members cause plant diseases known as rusts.

An order of fungi known as plant rusts that belong to the division (phylum) Basidiomycota. In nature, all 7000 species are obligate parasites of many vascular plant species. They cause diseases known as rust on numerous cultivated crops, and many trees are also attacked. Each rust species infects one or just a few closely related plant host species. Rust fungi occur on all continents except Antarctica.

The body of a rust fungus consists of numerous microscopic, threadlike, branching hyphae that grow inside the tissues and between the cells of the host plant. Specialized feeding structures, haustoria, grow from these hyphae into the host cells of the plant and provide nourishment for the hyphae. Hyphal cells are binucleate. Depending upon the rust species and environmental conditions, up to six different kinds of spore-producing structures, the sori, may be produced by one rust species. These sori may be powdery, waxy, or crustlike, and whitish, yellow, orange, brown, or blackish.

Rust teliospores that are produced in sori called telia are essential elements for classification. Teliospores consist of one or more specialized probasidial cells. During the development of each of these cells, nuclei fuse and meiosis occurs, resulting in a septate metabasidium from which four haploid meiospores (basidiospores) are formed. Basidiospores are forcibly ejected from the tips of their tiny stalks and are wind-disseminated. After landing on a susceptible host, under proper conditions basidiospores produce new infections. In cold regions, a rust may survive the winter as dormant, thick-walled, dark-colored teliospores. In many rust species, teliospores mature and produce basidiospores with no or little dormancy, especially in the tropics.

Although most rust species require only one host species to complete their life cycles (autoecious rusts), some of the best-known rusts require two taxonomically unrelated hosts (heteroecious rusts).

Rust diseases are controlled most effectively by breeding resistant host varieties. Fungicides are used for some rusts. In the case of some heteroecious rusts, eradication of one of the hosts (the noneconomic one) has been successful. Some rust species are used to aid in biological control of weeds. See also Basidiomycota; Eumycota; Fungi; Plant pathology.