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A class of mitosporic or anamorphic (asexual or imperfect) fungi belonging to the Deuteromycotina. They lack locular fruit bodies (conidiomata), and so sporulation occurs on separate or aggregated hyphae, which may or may not be differentiated; the thallus consists of septate hyphae. About 1400 genera comprising more than 11,500 species are recognized.
The Hyphomycetes, like other groups of Deuteromycetes, is an artificial one composed almost entirely of anamorphic fungi of ascomycete affinity. The majority are known anamorphs of Ascomycetes, although some have basidiomycete affinities. Several of the latter are aquatic or aero-aquatic. Taxa are referred to as form genera and form species, because the absence of a sexual or perfect teleomorph state forces classification and identification by artificial rather than phylogenetic means. The unifying feature of the group is the production of conidia from superficial, exposed conidiogenous cells arising separately from vegetative hyphae or cells (mononematous), or incorporated on conidiophores that may be entirely separate or aggregated in cushion-like sporodochia or stalk-like synnemata. Differences in insertion and arrangement of conidiogenous cells and conidiophores traditionally have been used to separate three orders. In the Hyphomycetales they are solitary or at most fasciculate and tufted; in the Tuberculariales they are produced over the outer surface of a cushion-shaped-to-flattened conidioma (sporodochium), and in the Stilbellales they are united into a stipitate conidioma (synnema). An alternative means of classifying hyphomycetes is based on differences in the ways that conidia are produced and conidiogenous cells grow before, during, and after conidiogenesis. See Ascomycota, Basidiomycota, Coelomycetes
To the naked eye, hyphomycete colonies are conspicuous as black, brown, green, gray, and white growths on substrata. In size, hyphomycete conidia vary from the minute to extremely long or wide. Shapes vary markedly within and between genera. Many hyphomycetes produce conidia in mucilaginous matrices. As in the Coelomycetes, the matrix inhibits or retards germination until the conidia become dispersed, and maintains germinability during periods of environmental stress. Other genera produce conidia in powdery masses, such hydrophobic conidia being more suited for air dispersal. Sterile elements in the form of simple or branched setae are commonly present among conidiophores or on conidiomata, and since they are particularly common among leaf-litter fungi they are thought to function as a form of predator defense.
The Hyphomycetes draw nourishment from living or dead organic matter and are adapted to grow, reproduce, and survive in a wide range of ecological situations. They can also be either stress-tolerant or combative. Some species grow among rubbish, and because their thin-walled, hyaline vegetative and reproductive structures make them more prone to attack and decay, they are ephemeral. Their ability to colonize, decompose, and use substrates and to interact with or parasitize other organisms is a result of the enzymes, antibiotics, toxins, and other metabolites they produce, coupled with wide genetic diversity. They are extremely common in soils of all types and on leaf litter and other organic debris of both natural and manufactured origin. They also cause extensive problems in food spoilage and occur in saline, stagnant, and fresh water.
Some hyphomycetes are found on or associated with fungi, including pathogens such as Verticillium, Mycogone, and Cladobotryum, and on lichens. Several are of medical importance, being associated with superficial, cutaneous, subcutaneous, and systemic infections. They are often opportunistic organisms that cause infections in immunocompromised patients. Toxic metabolites, or mycotoxins, are formed by many hyphomycetes, notably Aspergillus, Fusarium, and Penicillium. Others are nematophagous, capturing or consuming nematodes and other microfauna. Beauveria, Metarhizium, Hirsutella, and Entomophthora, for example, have been exploited for insect control. Many also cause economically important diseases in all types of vascular plants, especially agricultural and forestry crops. Hyphomycetes are primary pathogens of plants and weeds, causing root, stem, and leaf necrosis; diebacks; cankers; wilts; and blights. By infesting or contaminating seed, they can transmit seed-borne defects or reduce seed viability. See Medical mycology, Mycotoxin, Plant pathology
Hyphomycetes produce a wide variety of primary and secondary metabolites and are capable of effecting many different chemical and biochemical changes. By harnessing that capability in industrial processes, organic acids, enzymes, antibiotics, growth substances, alcohol, and cheese, among others, can be produced and steroid transformation can take place. See Deuteromycotina, Fungi