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The fungi imperfecti or imperfect fungi are fungi which do not fit into the commonly established taxonomic classifications of fungi that are based on biological species concepts or morphological characteristics of sexual structures because their sexual form of reproduction has never been observed. They are known as imperfect fungi because only their asexual and vegetative phases are known. They have asexual form of reproduction, meaning that these fungi produce their spores asexually, in the process called sporogenesis.
There are about 25,000 species that have been classified in the phylum Deuteromycota and many are Basidiomycota or Ascomycota anamorphs. Fungi producing the antibiotic penicillin and those that cause athlete's foot and yeast infections are algal fungi. In addition, there are a number of edible imperfect fungi, including the ones that provide the distinctive characteristics of Roquefort cheese and Camembert cheese.
Other, more informal names besides phylum Deuteromycota (or class "Deuteromycetes") and fungi imperfecti are anamorphic fungi, or mitosporic fungi, but these are terms without taxonomic rank. Examples are Alternaria, Colletotrichum, Trichoderma etc. The class Phycomycetes ("algal fungi") has also been used.
Mycologists formerly used a unique dual system of nomenclature in classifying fungi, which was permitted by Article 59 of the International Code of Botanical Nomenclature (the rules governing the naming of plants and fungi). However, the system of dual nomenclature for fungi was abolished in the 2011 update of the Code.
Under the former system, a name for an asexually reproducing fungus was considered a form taxon. For example, the ubiquitous and industrially important mold, Aspergillus niger, has no known sexual cycle. Thus Aspergillus niger was considered a form taxon. In contrast, isolates of its close relative, Aspergillus nidulans , revealed it to be the anamorphic stage of a teleomorph (the ascocarp or fruiting body of the sexual reproductive stage of a fungus), which was already named Emericella nidulans. When such a teleomorphic stage became known, that name would take priority over the name of an anamorph (which lacks a sexual reproductive stage). Hence the formerly classified Aspergillus nidulans would be properly called Emericella nidulans – note there's no reference to the original author. The system since 2013 instead treats both as the same species typified by the anamorph, and hence the author citation would include the original author as Emericella nidulans
However, because phylogenetic methods require sufficient quantities of biological materials (spores or fresh specimens) that are from pure (i.e., uncontaminated) fungal cultures, for many asexual species their exact relationship with other fungal species has yet to be determined. Under the current system of fungal nomenclature, teleomorph names cannot be applied to fungi that lack sexual structures. Classifying and naming asexually reproducing fungi is the subject of ongoing debate in the mycological community.
Following, a classification of the Fungi imperfecti: Saccardo et al.(1882-1972)
Other, according to Dörfelt (1989):Dörfelt, Heinrich (Hrsg.): Lexikon der Mykologie. Gustav Fischer Verlag, Stuttgart, New York. 1989.
Other systems of classification are reviewed by .
Nieuwenhuis and James (2016) also discusses the costs and benefits for sexual and reproduction in fungi as well as mechanisms that have evolved to reduce the costs of either. They also describe the so-called "parasexual cycle" in some fungi imperfecti, which allows recombination without sex.
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