|Disease of cucumber caused by Rhizoctonia solani|
Rhizoctonia is a genus of anamorphic fungi in the order Cantharellales. Species do not produce spores, but are composed of hyphae and sclerotia (hyphal propagules) and are asexual states of fungi in the genus Thanatephorus. Rhizoctonia species are saprotrophic, but are also facultative plant pathogens, causing commercially important crop diseases. They are also endomycorrhizal associates of orchids. The genus name was formerly used to accommodate many superficially similar, but unrelated fungi.
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 (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 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 (teleomorph Thanatephorus cucumeris), 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.
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 family Ceratobasidiaceae.
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 bataticola = Macrophomina phaseolina (Botryosphaeriaceae)
- Rhizoctonia carotae = Fibulorhizoctonia carotae (teleomorph Athelia arachnoidea)(Atheliaceae)
- Rhizoctonia cerealis = not a validly published name (teleomorph Ceratobasidium cereale) (Ceratobasidiaceae)
- Rhizoctonia crocorum = Thanatophytum crocorum (teleomorph Helicobasidium purpureum) (Helicobasidiaceae)
- Rhizoctonia fragariae = not a validly published name (teleomorph Ceratobasidium cornigerum) (Ceratobasidiaceae)
- Rhizoctonia goodyerae-repentis = a name of uncertain application, but generally interpreted as Ceratobasidium cornigerum (Ceratobasidiaceae)
- Rhizoctonia leguminicola = Botrytis fabae (Sclerotiniaceae)
- Rhizoctonia oryzae = not a validly published name and of uncertain application, but generally interpreted as Waitea circinata (Corticiaceae)
- Rhizoctonia ramicola = Ceratorhiza ramicola (teleomorph Ceratobasidium ramicola) (Ceratobasidiaceae)
- Rhizoctonia rubi = not a validly published name and of uncertain application (probably Ascomycota)
- Rhizoctonia zeae = Waitea circinata (Corticiaceae)
Habitat and distribution
Species are saprotrophic, occurring in the soil and producing basidiocarps (fruit bodies of the Thanatephorus teleomorph) 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.
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, sheath blight of rice, and many other pathogenic conditions.
- Andersen TF, Stalpers JA. (1994). "A checklist of Rhizoctonia epithets". Mycotaxon. 51: 437–457.
- Moore RT. (1987). "The genera of Rhizoctonia-like fungi". Mycotaxon. 29: 91–99.
- International Code of Botanical Nomenclature, Appendix III http://ibot.sav.sk/icbn/main.htm
- Moncalvo J-M; et al. (2006). "The cantharelloid clade: dealing with incongruent gene trees and phylogenetic reconstruction methods". Mycologia. 98 (6): 937–948. doi:10.3852/mycologia.98.6.937. PMID 17486970. http://www.endophytes.org/teaching/advmycol/Cantherelloid.Moncalvo.pdf
- Adams GC, Kropp BR. (1996). "Athelia arachnoidea, the sexual state of Rhizoctonia carotae, a pathogen of carrot in cold storage". Mycologia. 88 (3): 459–472. doi:10.2307/3760886. JSTOR 3760886.
- Roberts P. (1999). Rhizoctonia-forming fungi. Kew: Royal Botanic Gardens. p. 239. ISBN 1-900347-69-5.
- Rhizoctonia disease of potato http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_Rhizoctonia.htm
- Rhizoctonia root rot http://cbarc.aes.oregonstate.edu/rhizoctonia-root-rot-bare-patch
- Rhizoctonia diseases of sugar beet http://www.uiweb.uidaho.edu/sugarbeet/Disease/rtdspnw.htm
- Rhizoctonia disease of cucumber http://cuke.hort.ncsu.edu/cucurbit/cuke/dshndbk/br.html
- Rhizoctonia sheath blight http://www.lsuagcenter.com/NR/rdonlyres/C93A494B-8105-4804-9DFA-81190EC3F68B/58166/pub3123ShealthBlightofRiceHIGHRES.pdf
- Efficient Conversion of L-Tryptophan to Indole-3-Acetic Acid and/or Tryptophol by Some Species of Rhizoctonia. Toshiko Furukawa, Jinichiro Koga, Takashi Adachi, Kunihei Kishi and Kunihiko Syono, Plant Cell Physiol., 1996, volume 37, issue 7, pages 899-905 (abstract)