Rhizoctonia was introduced in 1815 by French mycologist Augustin Pyramus de Candolle for plant pathogenic fungi that produce both hyphae and sclerotia. "Rhizoctonia" means "root killer" and de Candolle's original species, Rhizoctonia crocorum, 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 comprising a diverse range of unrelated species. As part of a move towards a more natural classification of fungi, R.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 Botanical Nomenclature.
Current status
In its current sense, the genus is effectively restricted to the type species R. solani and its synonyms. The genus name is still, however, widely used in its old, artificial sense. Molecular research, based on cladistic analysis of DNA sequences, places Rhizoctonia within the familyCeratobasidiaceae.
Redisposition of former species
A comprehensive survey and redisposition of 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 fragariae = not a validly published name
Rhizoctonia goodyerae-repentis = a name of uncertain application, but generally interpreted as Ceratobasidium cornigerum
Rhizoctonia leguminicola = Botrytis fabae
Rhizoctonia oryzae = not a validly published name and of uncertain application, but generally interpreted as Waitea circinata
Rhizoctonia ramicola = Ceratorhiza ramicola
Rhizoctonia rubi = not a validly published name and of uncertain application
Rhizoctonia zeae = Waitea circinata
Habitat and distribution
Species are saprotrophic, occurring in the soil and producing basidiocarps 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.
