Uredinalis

(rust fungi), an order of basidiomycetous fungi. The fungi are obligate parasites of angiosperms, gymnosperms, and ferns and cause a plant disease known as rust. The endophytic, intercellular mycelium has haustoria that work their way into the cell bodies and supply the fungus with nutritive substances. In most species of rust fungi the mycelial cells and, especially, the spores contain a large quantity of oil; the droplets of oil are colored with orange or reddish brown lipochrome. The mycelium is usually concentrated at the site of penetration by the infection. After the formation of sporiferous structures, the fungus usually causes the death of the host plant’s tissues; the fungus itself dies soon afterward. Some rust fungi have a diffuse, often perennial mycelium that permeates the tissue of the entire plant. Such a mycelium often causes significant alteration in the anatomic structure of the infected organs, producing abnormal growths known as witches’-brooms.

The developmental cycle of rust fungi is characterized by the alternation of haploid and diploid stages. There are five types of sporiferous structures: the pycnium, which produces pycnio-spores; the aecium, which produces aeciospores; the uredini-um, which produces uredospores; the telium, which produces teliospores; and the basidium, which produces basidiospores. The pycnium and aecium belong to the haploid phase, whereas all the other structures are diploid.

Some species, known as complete species, include in their developmental cycle all the sporiferous forms; others, called incomplete species, include only a few. Incomplete species evidently arose from complete species as a result of the shortening of the developmental cycle under ecological conditions in which plants quickly complete their vegetative period. For this reason, incomplete species predominate in high-mountain and arctic regions; those found in temperate latitudes parasitize short-lived plants. Rust fungi whose sporiferous forms all develop on the same plant are called single-host fungi (for example, Puccinia helianthi—the cause of rust in sunflowers). Fungi that undergo their haploid phase on one plant species and their diploid phase on another are called multiple-host fungi (for example, P. graminis tritici—the cause of wheat rust—begins its development on barberries).

Rust fungi are highly specialized parasites. Some have a large number of special forms and physiological races that develop only on plants of certain genera, species, and even varieties. The fungus P. recondita tritici, which attacks wheat, comprises 228 physiological races. The two families Melampsoraceae and Pucciniaceae embrace 126 genera, with more than 5,000 widely distributed species.

Rust fungi cause rust in many cultivated crops and in a number of economically important wild plants. The species Melampsora infects conifers and flax, the species Cronartium conifers, the species Puccinia purse crops and sunflowers, the species Uromyces legumes, the species Gymnosporangium pears, the species Phragmidium roses and raspberries, and the species Hemileia coffee trees.