Rhizoctonia

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.[2] Subsequent authors added over 100 additional names to the genus, most of them plant pathogens bearing only a superficial resemblance to the type species.[3] Rhizoctonia thus became an artificial form genus of anamorphic fungi comprising a diverse range of unrelated species.[4]

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.[5] 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.[6]

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.[5]

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).[7] In 1996, on the basis of their similar morphology, the two genera were considered to be synonymous by K. Hauerslev and P. Roberts.[8] in 1970 Svrček & Pouzar introduced the genus Cejpomyces for a species resembling Thanatephorus, but having septate basidiospores.[9] Based on their similar morphology, the genera were considered synonymous by Langer (1994).[8] In 1971 Talbot & Keane introduced the genus Oncobasidium for a plant pathogenic species resembling Thanatephorus but lacking sclerotia[10] and in 1972 M.A. Donk introduced the genus Ypsilondium for a species resembling Uthatobasidium but having bisterigmate (two-spored) basidia.[11] Both genera were considered synonyms of Thanatephorus by Roberts (1999).[8] In 1978 Tu & Kimbrough introduced the genus Aquathanatephorus for an isolate from water hyacinth which produced a teleomorph with swollen, inwardly curving sterigmata.[12] This was redetermined as Thanatephorus cucumeris by Andersen (1996).[13]

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.[14] 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.[15] Talbot (1965) demonstrated that such spores were present in some collections and suggested that Koleroga be synonymized with Ceratobasidium.[8]

Molecular research, based on cladistic analysis of DNA sequences, places Rhizoctonia within the family Ceratobasidiaceae.[16][17] The genus is only monophyletic, however, if species of Ceratobasidium (excluding the type)[18] and Ceratorhiza are included as synonyms, since there is no apparent distinction between these species and species of Rhizoctonia.[17][18] DNA sequencing has also confirmed the synonymy of Uthatobasidium, Oncobasidium, and Koleroga.[18]

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.[18] In its current sense, therefore, the genus Rhizoctonia includes both anamorphic and teleomorphic fungi. Not all species referred to Ceratobasidium or Thanatephorus have yet been combined in Rhizoctonia, however.

A comprehensive survey and redisposition of old species names in Rhizoctonia was published in 1994 by Andersen & Stalpers.[4] Only a few frequently used names are listed below. Many older names are of uncertain application or were never validly published, or both.[4]

• Rhizoctonia bataticola = Macrophomina phaseolina (Botryosphaeriaceae)[3]
• Rhizoctonia carotae = Athelia arachnoidea (Atheliaceae)[19][3]
• Rhizoctonia crocorum = Helicobasidium purpureum (Helicobasidiaceae)[4]
• Rhizoctonia leguminicola = Botrytis fabae (Sclerotiniaceae)[4]
• Rhizoctonia oryzae = not a validly published name but now validated as Waitea oryzae (Corticiaceae)[3]
• Rhizoctonia rubi = not a validly published name and of uncertain application (probably Ascomycota)[4]
• Rhizoctonia zeae = Waitea zeae (Corticiaceae)[3]