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Æcidiomycetes (ē-sĭd′ĭ-ō-mī-se′-tēz), the technical name of a large group of plants commonly known as "rusts" and "smuts." They are very destructive parasites and attack some of the most valuable crops. The rusts chiefly attack the leaves of the higher plants, producing rusty lines and dots; while the smuts chiefly prey upon the grasses and are very injurious to cereals, attacking the grains of wheat, corn, oats, barley, etc. These parasites are very polymorphic, that is they assume different forms at different periods in their life histories, and these different phases have often been described as different kinds of plants. The history of these parasites is often further complicated by the fact that in their different phases they may live on different plants (hosts). Great attention has been paid to these destructive forms by the national and state governments in the hope that some way may be discovered by which their destructiveness may be lessened. The wheat rust is one of the best known forms, and its life history may serve as an illustration of the whole group. When the wheat is growing, rusty lines and dots appear on the leaves and stalk. These spots are known as the "red rust," and are masses of spores which have come to the surface, and are called "summer spores" or "uredospores." (III ur.) They arise from the threadlike body or "mycelium" of the parasite, which is imbedded out of sight among living cells of the wheat body and is feeding upon them. These summer spores are easily scattered by the wind, and falling upon other wheat plants germinate, enter the body, and begin to feed upon it. By means of the summer spores, therefore, the rust disease is spread rapidly during the growing season. In the late summer and fall black lines and dots appear upon the wheat stubble, forming the so-called "black rust," being masses of peculiar spores called "teleutospores" or "winter spores," because they last through the winter. (II, t.) These teleutospores come from the same mycelium (white threads) which produced the summer spores, and are the last spores it produces. Early the following spring the teleutospores germinate and form a little filament which produces very minute spores called "sporidia." These sporidia are scattered by the wind, and falling upon barberry leaves they germinate, the mycelium entering the leaf. This new mycelium sends to the surface of the leaf, especially the under surface, masses of orange-colored spores in little cup-like clusters, which are called "cluster-cups," or each is technically called an "æcidium." (I, a.) These æcidium spores from the barberry are carried by the wind to the young wheat, where they germinate and produce the mycelium with which we started. In case the barberry is not present to be used as a host for the æcidium stage this stage is omitted, and the sporidia pass directly to the wheat and germinate. Tnere are thus three distinct phases in the life history of this plant, at least four kinds of spores, and two host plants, the sporidium phase not being a parasite. See Fungi.

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Phases of wheat rust:
(I) cluster-cups on barberry (lower side);
(II) winter spores (teleutospores) on wheat;
(III) summer spores (uredospores) on wheat.