Thanatephorus Cucumeris

''Rhizoctonia'' is a genus of fungi in the order Cantharellales. Species form thin, effused, corticioid basidiocarps (fruit bodies), but are most frequently found in their sterile, anamorphic state. ''Rhizoctonia'' species are saprotrophic, but some are also facultative plant pathogens, causing commercially important crop diseases. Some are also endomycorrhizal associates of orchids. The genus name was formerly used to accommodate many superficially similar, but unrelated fungi.

Taxonomy

History

Anamorphs

''Rhizoctonia'' was introduced in 1815 by French mycologist Augustin Pyramus de Candolle for anamorphic plant pathogenic fungi that produce both hyphae and sclerotia. The name is derived from Ancient Greek, ῥίζα (''rhiza'', "root") + κτόνος (''ktonos'', "murder"), and de Candolle's original species, ''Rhizoctonia crocorum'' (teleomorph '' Helicobasidium purpureum''), is the causal agent of violet root rot of carrots and other root vegetables. Subsequent authors added over 100 additional names to the genus, most of them plant pathogens bearing only a superficial resemblance to the type species. ''Rhizoctonia'' thus became an artificial form genus of anamorphic fungi comprising a diverse range of unrelated species.

As part of a move towards a more natural classification of fungi, American mycologist Royall T. Moore proposed in 1987 that ''Rhizoctonia'' should be restricted to the type species and its relatives, with unrelated species moved to other genera. Unfortunately, this meant that the best-known but unrelated species, ''Rhizoctonia solani'', would have undergone a name change to ''Moniliopsis solani''. To avoid this, it was subsequently proposed that ''R. solani'' should replace ''R. crocorum'' as the type species of ''Rhizoctonia''. This proposal was passed and the type of ''Rhizoctonia'' is now conserved as ''R. solani'' under the International Code of Nomenclature for algae, fungi, and plants.

R.T. Moore retained species having teleomorphs in the genus ''Thanatephorus'' within ''Rhizoctonia'', but moved those with teleomorphs in the genus ''Ceratobasidium'' to the new anamorphic genus ''Ceratorhiza''.

Teleomorphs

In 1956, Dutch mycologist M.A. Donk published the new teleomorphic genera ''Thanatephorus'' and ''Uthatobasidium'' simultaneously, reserving the former for plant-pathogenic species producing sclerotia-bearing ''Rhizoctonia'' anamorphs (with ''T. cucumeris'' as the type) and the latter for saprotrophic species not producing anamorphs (with ''U. fusisporum'' as the type). In 1996, on the basis of their similar morphology, the two genera were considered to be synonymous by K. Hauerslev and P. Roberts. in 1970 Svrček & Pouzar introduced the genus ''Cejpomyces'' for a species resembling ''Thanatephorus'', but having septate basidiospores. Based on their similar morphology, the genera were considered synonymous by Langer (1994). In 1971 Talbot & Keane introduced the genus ''Oncobasidium'' for a plant pathogenic species resembling ''Thanatephorus'' but lacking sclerotia and in 1972 M.A. Donk introduced the genus ''Ypsilondium'' for a species resembling ''Uthatobasidium'' but having bisterigmate (two-spored) basidia. Both genera were considered synonyms of ''Thanatephorus'' by Roberts (1999). In 1978 Tu & Kimbrough introduced the genus ''Aquathanatephorus'' for an isolate from water hyacinth. This was redetermined as ''Thanatephorus cucumeris'' by Andersen (1996).

''Ceratobasidium'' was introduced in 1935 by American mycologist D.P. Rogers to accommodate species of the old form genus '' Corticium'' that showed affinities with the heterobasidiomycetes. These affinities were the possession of large sterigmata ("cerato-basidium" means "horned basidium") and the production of basidiospores that produce secondary spores. The genus ''Koleroga'' was proposed by Donk (1958) to accommodate ''K. noxia'', a plant pathogen morphologically similar to ''Ceratobasidium'' but not known to produce secondary spores. Talbot (1965) demonstrated that such spores were present in some collections and suggested that ''Koleroga'' be synonymized with ''Ceratobasidium''.

Current status

Molecular research, based on cladistic analysis of DNA sequences, places ''Rhizoctonia'' within the family Ceratobasidiaceae. The genus is only monophyletic, however, if species of ''Ceratobasidium'' (excluding the type) and ''Ceratorhiza'' are included as synonyms, since there is no apparent distinction between these species and species of ''Rhizoctonia''. DNA sequencing has also confirmed the synonymy of ''Uthatobasidium'', ''Oncobasidium'', and ''Koleroga''.

Following changes to the International Code of Nomenclature for algae, fungi, and plants, the practice of giving different names to teleomorph and anamorph forms of the same fungus was discontinued, meaning that ''Thanatephorus'' became a synonym of the earlier name ''Rhizoctonia''. In its current sense, therefore, the genus ''Rhizoctonia'' includes both anamorphic and teleomorphic fungi.

Redisposition of former species

A comprehensive survey and redisposition of old species names in ''Rhizoctonia'' was published in 1994 by Andersen & Stalpers. Only a few frequently used names are listed below. Many older names are of uncertain application or were never validly published, or both.

*''Rhizoctonia bataticola'' = ''Macrophomina phaseolina'' (Botryosphaeriaceae)
*''Rhizoctonia carotae'' = '' Athelia arachnoidea'' ( Atheliaceae)
*''Rhizoctonia crocorum'' = '' Helicobasidium purpureum'' (Helicobasidiaceae)
*''Rhizoctonia leguminicola'' = '' Botrytis fabae'' ( Sclerotiniaceae)
*''Rhizoctonia oryzae'' = not a validly published name but now validated as '' Waitea oryzae'' (Corticiaceae)
*''Rhizoctonia rubi'' = not a validly published name and of uncertain application (probably Ascomycota)
*''Rhizoctonia zeae'' = '' Waitea zeae'' (Corticiaceae)

Habitat and distribution

Species are saprotrophic, often occurring in soil and producing basidiocarps (fruit bodies) on dead stems and plant detritus. They are also opportunistic plant pathogens, with an almost unlimited host range, and have been isolated from orchid mycorrhiza. Distribution appears to be cosmopolitan.

Economic importance

''Rhizoctonia solani'' causes a wide range of commercially significant plant diseases. It is one of the fungi responsible for Brown patch (a turfgrass disease), damping off in seedlings, as well as black scurf of potatoes, bare patch of cereals, root rot of sugar beet, belly rot of cucumber

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