UREDINALES, (Neo-Lat. nom.
pl., from Lat. credo, blight, blast, from urere, Slat. us, to burn). A group of fungi familiarly called 'rusts' which live as parasites on flower ing plants. It includes very many species, which have complicated and diverse life-histories, but are identified by the teleutospore stage, this being normally a resting period in the life history. The teleutospores are formed after a period of active vegetation and previous to some unfavorable season, as winter. They are one to several-celled, and exhibit such a variety in structure that they serve to distinguish the various genera. After the return of a favorable season each cell of the teleutospore produces a short filament (promyeelium), each of whose cells. generally four in number. develops a small spore (sporidium) which serves to infect the proper host. There are some rusts, confined to one host, that never produce more than two forms of spores (teleutospores and sporidia), that alter nate with one another. Most rusts, however, have other forms of spores. Conspicuous them are the uredospores, somewhat similar to teleutospores. They are one-celled, and are formed during the vegetative season before the teleutospores. They are especially conspicuous on some species of Puecinia that grow on oats and wheat, and most of the so-called 'rust' of these cereals is due to the development of im mense numbers of uredospores colleeted in spots and lines along the leaves and stems. The most complicated life-histories, however, include still another form of fructification (fecidium or 'clus ter-cup'), in which the spores are found in paral lel chains at the bottom of a cup-like structure.
The cluster-cups are generally found on a dif ferent host plant from the teleutospores, so that such rusts must depend on two hosts for the completion of their life-history. For example, in the well-known wheat rust (Pueeinia grami vis) the sporidia germinate on the barberry. pro ducing there the cluster-cups (mcidia) ; while the mcidial spores are carried to wheat, oats, and other cereals, where several generations of ure dospores (red rust) may be developed. The ure dospores, being scattered, germinate on other individuals, spreading the rust Very rapidly. Finally, at the end of the season teleutospores (black ruse) are formed. Another interesting life-history is that of the apple rust. (See APPLE, paragraph Discuses.) Urcdospores may survive unfavorable seasons and propagate rust the following year. This probably accounts for the abundance of wheat rust, in the Western United States and Australia, where there are no wild barberries. In these regions the bar berry (cluster-cup) phase of the life-history is omitted, and the uredospores, or perhaps the sporidia, germinate directly upon the wheat. The rust is due to certain species of Puccinia whose eluster-cups are on plants other than the bar berry.
Consult: Engler and Prantl, Die natiirliehen Pflanzenfamilien (Leipzig, 1887—) ; Plowright, British Ureclinew and Ustilaginew.