Rhizoctonia DC.

Rhizoctonia is a genus of fungi in the family Ceratobasidiaceae. Species form thin, effused, corticioid basidiocarps (fruit bodies), but are most frequently found in their sterile, anamorph 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. As of 2025, the synonymisation of several genera under a unified Rhizoctonia was formalised, and the genus now includes names formerly circumscribed under Moniliopsis, Ceratobasidium, Thanatephorus, Uthatobasidium, Koleroga, Cejpomyces, Oncobasidium, Ypsilonidium, Ceratorhiza, and Tofispora. As currently circumscribed, the genus contains 52 accepted species.

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 which produced a teleomorph with swollen, inwardly curving sterigmata. This was redetermined as Thanatephorus cucumeris by Andersen (1996).

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