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(smut fungi), an order of basidiomycetous fungi. These fungi are parasitic on higher plants and cause diseases known as smuts. They are especially harmful to cereals. Altogether, some 700 species are known (on cultivated and wild plants), making up 42 genera. Representatives of this order are found in all parts of the world, including the high latitudes and mountainous regions.
The infection of plants remains unnoticed for some time because the development of the mycelium, diffusely distributed within the host plant, does not cause external changes. The fungus mycelium grows rapidly only prior to spore formation. The fungus hyphae are divided by transverse septa into sections of more or less equal length and width. The covering of these sections becomes mucilaginous, acquires a particular structure that is either prickly or netted, and usually darkens considerably. In this manner smut spores develop, called chlamydospores after the process by which they were formed. The hyphae from which the smut spores are formed disintegrate into single-cell chlamydospores (as in Tilletia and Ustilago) or into spore balls, that is, glomerules composed of spores gathered into a dense mass. In some genera all the cells of the spore balls are capable of development (as in Thecaphora and Tolyposporium), whereas in others only some may develop with the remaining cells acting as aerial sacks or floating bubbles and thereby enabling the spore ball to spread through the air (in Tuburcina) or by water (in Doas-sansia). Those parts of a plant where fungus chlamydospores usually form in great numbers are destroyed and in most cases appear to be charred or covered with soot (hence the name). Chlamydospores may form on flowers, leaves, or stems and in a few plants on roots. In every species of smut fungi, the chlamydospores may be found only in certain organs of the host plant. Thus, for example, in covered wheat smut chlamydospores occur only in the ovaries of the flowers and in stem rye or wheat smut only on the stems. In some species of the Caryophyllaceae family the chlamydospores appear only in the anthers of stamens; loose wheat smut destroys practically all parts of the ear, turning it into a dusty mass of spores and leaving only the stalk.
Ustilaginales are classified into two families, the Usti-laginaceae and the Tilletiaceae, according to the way in which the chlamydospores germinate. The germinating chlamydospores of fungi of the Ustilaginaceae family form a basidium that is divided transversely into four uninucleate cells (the so-called fragmobasidium). Along the sides of these cells there arise basidiospores in the form of outgrowths. The chlamydospores of fungi of the Tilletiaceae family produce a single-cell basidium with a group of threadlike basidiospores on the apex.
The reproductive process of smut fungi involves the copulation of two basidiospores or of the cells that have budded from them. The content of one spore is transferred to the other, but the nuclei do not join. From the diploid cell that forms, a mycelium with binucleate bells develops that is capable of infecting plants. Thus, the developmental cycle of smut fungi consists in an alternation of haploid (basidia and basidiospores) and diploid (mycelium and chlamydospores) phases. The most significant stage in the life of smut fungi is the diploid phase, which begins with the copulation of the basidiospores and ends with the reduction division of the nucleus in the chlamydospore.
REFERENCESGutner, L. S. Golovnevye griby. Moscow-Leningrad, 1941.
Kursanov, L. I., N. A. Komarnitskii. Kurs nizshikh rastenii, 3rd ed. Moscow, 1945.
Ul’ianishchev, V. I. Opredelitel’ golovnevykh gribov SSSR. Leningrad, 1968.
B. P. VASIL’KOV