Rhizoctonia solani

O’Donnell, R. P., Linde, C. C. & May, T. W., 2025, Rise up, Rhizoctonia: moving to one fungus, one name in the Ceratobasidiaceae (Agaricomycetes, Cantharellales), Persoonia 54 (1), pp. 285-325 : 313

Rhizoctonia solani J.G. Kühn, Die Krankheiten der Kulturgewächse, ihre Ursachen und Verbreitung (Berlin): 224. 1858 .

Synonyms: Hypochnus cucumeris A.B. Frank, Ber. Deutsch. Bot. Ges. 1: 62. 1883.

Thanatephorus cucumeris (A.B. Frank) Donk , Reinwardtia 3: 376. 1956.

Ceratobasidium solani (Prill. & Delacr.) Pilát, Česká Mykol. 11 (2): 81. 1957.

Moniliopsis solani (J.G. Kühn) R.T. Moore, Mycotaxon 29: 95. 1987.

Type citation: The citation in the protologue is: ‘Der von mir beobachtete Pilz ist ein, wie ich glaube, noch nicht beschriebene Gebilde, Rhizoctonia solani mihi … Man sieht der Pilz hier auf der Oberflache in Form einzelner, nicht sehr reich verzweigter, aber vielfach oft scharfeckig gebogener, dunkelbraunen Fäden, von Denen man bei verschiedener Einstellung nicht selten deutlich wahrnehmen kann, dass sie an ihren Ausgangspunkte aus der Rinde der Kartoffeln hervorkommen.’ [‘The fungus which I observed is one, which I believe, whose structures have not yet been described, is named by me Rhizoctonia solani … One observes the fungus on the surface in the individual form, not very richly branched, but often many sharply curved, dark brown threads, from which one can clearly observe from different angles, that they emerge from the rind of the potatoes.’]. The name is conserved with a conserved type: ‘ Netherlands, Aug 1995, Boogaert ([dried culture ex] CBS No. 739.95 “239.95”)’ (Typ. cons.).

ITS barcode: MH862557 ( T).

UNITE 1.5 % SH: SH0814567.10FU.

Ex-type culture: CBS 739.95.

Notes: Rhizoctonia solani is considered the asexual morph name of Th. cucumeris , and the two names are treated as synonymous ( Stalpers et al. 2021). Andersen & Stalpers (1994) and Roberts (1999) include many further synonyms under R. solani , but this would have been under a morphological concept. Given the few morphological characters available for the asexual morph, all synonyms need to be re-examined in the light of the phylogenetic species delimitation adopted herein. Names which require re-examination of type material to ascertain placement under current phylogenetically-defined species include R. napae Westend. & Wallays , [as ‘ napaeae ’], R. betae Eidam. , R. fusca Rostr. , R. mucoroides G.E. Bernard, R. potomacensis Wollenw. , R. grisea (J.A. Stev.) Matz , R. dimorpha Matz , R. ferruginea Matz , R. macrosclerotia Matz , R. melongenae Matz , R. gossypii var. anatolica Forsten. , R. gossypii var. egyptiaca Forsten. , R. gossypii Forsten. var. gossypii ., R. aderholdii (Ruhland) Marchion. , R. chousii Crand. & Arill. , and Th. corchori C.C. Tu et al.

In their proposal to conserve the names Rhizoctonia and R. solani, Stalpers et al. (1998) cite the CBS culture CBS 239.95 as a proposed conserved type. The culture CBS 239.95 is currently identified as Amoenomyces catenosporus . However, the entry for R. solani in Appendix III of the Shenzhen Code indicates that the accession number should be corrected to CBS 739.95 ( Wiersema et al. 2018).

Among the four major AG groups delimited by Boidin et al. (1998), the following set were placed under R. solani (as Th. cucumeris ): AG-2, AG-3, AG-5, AG-6, AG-8 and AG- BI. Among these, Andersen & Stalpers (1994) and Roberts (1999) considered that AG-3 is “associated” with the names R. solani and Th. cucumeris . However, the ITS sequence generated from the conserved type culture for R. solani (CBS 739.95; GenBank MH862557 ) is clustered with sequences of AG- 5 in this study ( Fig. 3). Indeed, when BLASTed, the majority of close matches for this sequence are identified as AG-5. While Stalpers et al. (1998) may have intended to use an isolate of AG-3 to anchor the name R. solani , molecular data suggest that they may have instead selected an isolate of AG-5. With this considered, AG-5 should consequently be the AG name associated with the name R. solani . Careful examination of the types of the numerous un-assigned names in Rhizoctonia , in concert with generation of ex-type or reference sequences, is necessary to match up potential names to as yet un-named AG groups formerly placed under R. solani that can be shown to be phylogenetically distinct.