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a class of higher fungi with about 2,000 genera, embracing 15,000 species. Ascomycetes are characterized by a multicellular mycelium and special spore-bearing organs, or asci, in which ascospores develop. Usually eight spores develop in each ascus. The asci appear as a result of the sexual process, which varies in different ascomycetes. In most species the asci develop internally or on the surface of the fruiting bodies; in Endo-mycetales they develop directly on the mycelium or on the budding cells, without formation of a fruiting body. The classification of ascomycetes is based on the structure of the fruiting bodies and the asci.
Many ascomycetes, including Plectascales, Perisporiales, and Pyrenomycetes, also reproduce asexually by means of conidia; this form of reproduction precedes the formation of asci at the end of the developmental cycle. In many fungi asci seldom develop; these fungi reproduce almost exclusively by means of conidia. A fungus of which only the conidial stage is known is often described as an independent, or imperfect, fungus. An ascomycete that produces conidia under one set of conditions often may not produce them under another.
Most ascomycetes live as saprophytes in soil, on dead plant tissues, on substrates of organic origin (manure, skin, hair), in food products, and in fermenting liquids. Ascomycetes include parasites of higher plants and the causative agents of mycoses in animals and humans. The vegetative period of some species is spent in the conidial stage as parasites on living plants; after the death of the plants the fungi transfer to saprophytic nutrition, forming spore-bearing asci by spring. Almost all fungi that participate in the formation of lichens are ascomycetes.
Many ascomycetes are the causative agents of diseases of cultivated and beneficial wild plants. They cause powdery mildew, canker, spot, scab, snow mold, and root rot. The conidial stages of many species cause spoilage of food products and feeds. Certain types of mold fungi (the genus Penicillium) are used in the production of cheese, bread, and antibiotics. Some ascomycetes are edible, for example, morels and truffles.
V. A. MEL’NIK