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Remarks (internal):Thanatephorus is distinguished by the hymenium consisting of successive layers of basidia borne on vertically branching hyphae arising in cymes from the basal hyphae, ellipsoid to oblong, terminal basidia, sometimes swollen but typically less than twice the width of the subtending hypha at the sub-basidial septum, and septal pores with discontinuous parenthesomes.
The genus is morphologically close to Ceratobasidium, but differs in its vertically branching hymenium producing successive layers of basidia. Basidia are consequently terminal and more elongated than in Ceratobasidium, with a length to width ratio (Q) between 1.2 and 3.2. The average ratio is above 1.4, which is the maximum ratio for basidia of Ceratobasidium (Table 3). Thanatephorus also has wider hyphae than Ceratobasidium, typically 6-8 µm diam. and sometimes up to 18 µm diam., though this is an observation based on known species rather than a defining characteristic of the genus.
Thanatephorus is also morphologically close to Botryobasidium, a holobasidiomycetous genus with which it was originally united (Donk, 1931). Botryobasidium has a similar hymenial structure, but differs in its short-sterigmate basidia, lack of basidiospore replication, and lack of monilioid hyphae or sclerotia. Langer (1994) showed that
Botryobasidium has septal pores with continuous parenthesomes, whereas Thanatephorus and Ceralobasidium have discontinuous parenthesomes.
Donk (1956) published the new genera Thanatephorus and Uthatobasidium simultaneously, reserving the former for plant-pathogenic species producing sclerotia (with T cucumeris as the type) and the latter for saprotrophic species not producing sclerotia (with U. fusisporum as the type). However, as noted in the introduction, parasitism in this group of fungi is facultative and T cucumeris often occurs as a saprotroph so that the main generic distinction is weak. It becomes untenable when additional species, such as Oncobasidium theobromae, are considered. In order to retain Donk's generic concepts, Talbot & Keane (1971) proposed their new genus Oncobasidium for plant pathogenic species resembling Thanatephorus but not producing sclerotia. On this basis, anastomosis groups of T cucumeris which are not known to be pathogenic or sclerotia-forming should be referred to Uthatobasidium, whilst others which are pathogenic but not known to produce sclerotia should be referred to Oncobasidium. These generic concepts are clearly unsound, and it is here proposed that Uthatobasidium and Oncobasidium be synonymized with Thanatephorus, in the absence of any other distinguishing morphological features between them. The synonymization of Uthatobasidium has been published by K. Hauerslev and the present author in Knudsen & Hansen (1996).
The genus Ypsilonidium was proposed by Donk (1972) to accommodate bisterigmate species previously referred to Thanatephorus (despite being non-pathogenic). The possession of bisterigmate basidia is not the sole criterion for generic separation in any other group of Basidiomycetes and is not applied, for example, in Ceratobasidium, where C. bicorne is bisterigmate. Accordingly, Ypsilonidium is considered a further synonym of Thanatephorus, as already proposed by Langer (1994).
The genus Cejpomyces was distinguished from Thanatephorus in that its type and only species, C. terrigenus, produced basidiospores which occasionally became septate and were not seen to be self-replicating. Jülich (1981) considered this sufficiently distinctive to propose a new family, Cejpomycetaceae, to accommodate the species. Langer (1994), however, placed Cejpomyces in synonymy with Thanatephorus, based on its overall morphology. This synonymy is accepted here.
Aquathanatephorus was proposed to accommodate an isolate from water hyacinth (Eichhornia), originally determined as Rhizoctonia solani, which produced a teleomorph with swollen, inwardly curving sterigmata (Tu & Kimbrough, 1978). The type and only collection is now lost (fide herb. in litt.), but an authentic culture of the anamorph has been redetermined as Thanatephorus cucumeris (Andersen, 1996) and Aquathanatephorus is here considered a synonym.
The genus Tofispora was proposed by Langer (1994) to accommodate three tropical species differing from Thanatephorus only by having warted rather than smooth basidiospores. In the same monograph, smooth, warted, and spiny-spored species were accepted in the single genus Botryobasidium, so that the criterion of spore decoration, as well as being a single characteristic, was not consistently applied. Since Tofispora is not otherwise distinct from Thanatephorus (Roberts, 1998e), it is here considered a further synonym.
Anamorphs of Thanatephorus have historically been assigned to Rhizoctonia. However, the type of Rhizoctonia is R. crocorum DC., the anamorph of Helicobasidium purpureum, a species with auricularioid basidia and simple septal pores. To establish a more natural classification, Moore (1987), following a proposal by Donk (1966), re-established Moniliopsis as the anamorph genus for rhizoctonias having septal pores with discontinuous parenthesomes and multinucleate hyphae (thus including the anamorphs of Thanatephorus cucumeris and Waitea circinata). The type species of Moniliopsis, M. aderholdii, is a synonym of R solani, the anamorph of Thanatephorus cucumeris.
However, because of the widespread use of the name Rhizoctonia solani, a proposal is being made (Stalpers et al., 1998) to amend the International Code of Botanical Nomenclature to conserve Rhizoctonia against Moniliopsis by changing its type species from R. crocorum to R. solani. An additional proposal is to conserve R. solani against R. napaeae, an earlier valid name for the species. These proposals are accepted here, in the anticipation that the amendments will be passed. Accordingly, Moniliopsis is here treated as a synonym of Rhizoctonia.
Description type:Non-original description 
Description:Thanatephorus Donk, Reinwardtia 3: 376 (1956).
Uthatobasidium Donk, Reinwardtia 3: 376 (1956).
Ceypomyces Svrcek & Pouzar, Ceská Myk. 24: 5 (1970).
Oncobasidium P.H.B. Talbot Sc Keane, Australian J. Bot. 19: 203 (1971).
Ypsilonidium Donk, Proc. K Ned. Akad. Wet. C75: 371 (1972).
Aquathanatephorus C.C. Tu & Kimbr., Bot. Gaz. 139: 459 (1978).
Tofispora G. Langer, Bib. Myc. 158: 32 (1994).
(Anamorph) Rhizoctonia DC., Mém. Mus. Hist. Nat. Paris 2: 216 (1815). [proposed as a nom. cons.]
Moniliopsis Ruhland, Arb. K Biol. Anst. 6: 76 (1908).
Teleomorph. Basidiome: effused, thin, hypochnoid to ceraceous (in one species subgelatinous), smooth, whitish to ochraceous. Hymenium: comprising a layer of basidia on vertically branching, cymose, thin-walled hyphae arising from a subicular layer of wider, thick-walled basal hyphae. Hyphae: bi-or multinucleate, subhymenial hyphae thin-walled basal, hyaline, with short hyphal compartments, somewhat swollen, (4-) 6-10 µm diam.; basal hyphae hyaline to ochraceous to brown, with long hyphal compartments, straight, (3-) 5-12 (-18) µm diam., often with thickened walls, often double-laminate. Clamp connexions absent. Septal pores: dolipores with discontinuous parenthesomes. Basidia: ellipsoid to oblong or cylindrical (Q= (1.2-) 1.4-2.8(-3.2)), (10-)12-20(-33) x (7-)8-12(-16) µm. Sterigmata: (1-)2-4(-8), (4.5-)5-10(-30) x 1.5-2.5(-5.5) µm, occasionally producing subsidiary sterigmata and then appearing furcate. Basidiospores: globose, ellipsoid, citriform, or oblong, hyaline to ochraceous, thin-to slightly thick-walled, producing secondary spores by replication.
Anamorph. Rhizoctonia anamorphs produce monilioid hyphae with variable, ellipsoid to subglobose compartments; sclerotia are also often produced.
Distribution and ecology. Cosmopolitan. Species are saprotrophic, plant parasitic, and orchid endomycorrhizal.
Type species. Thanatephorus cucumeris
(Anamorph) Rhizoctonia solani
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