Rhizoctonia hydrophila (Sacc.) R.P. O’Donnell, C.C. Linde & T.W. May, comb. nov. MB 852068.

Basionym: Sclerotium hydrophilum Sacc. Syll. fung. (Abellini) 14 (2): 1141. 1899.

Synonym: Ceratorhiza hydrophila (Sacc.) Z.H. Xu et al., Mycologia 102 (2): 340. 2010.

Type citation: ‘ap. Rothert Ueb. Sclerotium hydroph. Leipzig. 1892 (Bot. Zeit.)…Hab. in caulibus, quos vexat v. necat, Myriophylli et Hydrocharidis in horto bot. Strasbourg et Kazan, ubi legit cl. ROTHERT, qui inde coluit in compluribus aliis plantis.’ [‘In the writings of Rothert about Sclerotium hydroph. Leipzig. 1892 (Bot. Zeit.) ... Living in the stems, where it plagues or kills, Myriophyllum and Hydrocharis in the botanic gardens of Strasbourg and Kazan, where it was collected by ROTHERT, who later cultivated it in several other plants.’]. Lectotype (here designated, MBT 10018477), illustration in Rothert, W. (1892) Botanische Zeitung 50, Taf. VII (reproduced here as Fig. 5a and 5b). Epitype (here designated, MBT 10018479): The Netherlands, isolated from Victoria regia,1927, C. J. Buisman (CBS 201.27, culture stored in a metabolically inactive state).

ITS barcode: FJ231396 (T).

UNITE 1.5 % SH: SH0916688.10FU.

Alternative marker: LSU = FJ212349 (T).

Ex-epitype culture: CBS 201.27.

Notes: In their molecular analyses to determine the phylogenetic placement of Sclerotium hydrophilum, Xu et al. (2010) used sequences from CBS cultures CBS 201.27 (isolated from the water lily Victoria regia in The Netherlands) and CBS 385.63 (from a "submerged leaf in garden pond" in Italy). Both CBS-derived sequences were recovered in a clade closely related to sequences variously identified as Ceratorhiza, and S. hydrophylum was consequently transferred to Ceratorhiza. The name Ceratorhiza hydrophila has subsequently been used to identify further isolates from aquatic plants, including Myriophyllum spicatum (Elsaba et al. 2022). The name S. hydrophilum was originally introduced by Rothert (1892) in an extensive discussion of a sclerotiumforming fungus found in association with water plants. Rothert (1892) indicated that P.A. Saccardo examined material and provided the comment ‘Nous pouvez, je pense, le decrire sous un nom provisoire, p. e. Sclerotium hydrophilum ’ [‘We can, I think, describe it under a provisional name Sclerotium hydrophilum ’.] Due to the provisional nature of the name, as introduced by Rothert (1892), it is invalid according to Art. 36.1. Saccardo & Sydow (1899) later validly published the name, attributed there to P.A. Saccardo alone, and provided a description which cited Rothert (1892). Gola (1930) in his catalogue of the Saccardo Herbarium (now at PAD) does not list any material of S. hydrophilum. Hence, we designate as lectotype the illustration in Rothert (1892) because it can be considered part of the original material under Art. 9.4, being published illustrations that Saccardo (in Saccardo & Sydow, 1899) associated with the taxon via his citation of Rothert (1892). The illustration in Rothert (1892) depicts the internal structure of the sclerotium (as globose to irregularly subglobose to ellipsoid elements) along with cylindrical, septate hyphae, lacking clamp connections, some with right angled branching. These characters are consistent with Rhizoctonia but are not characteristic enough at species level because a number of species of Rhizoctonia form sclerotia (Sumner 1996). Because of this ambiguity, we therefore epitypify the name as above. A separate phylogenetic analysis of GenBank ITS sequences identified as Ceratorhiza hydrophila / Sclerotium hydrophilum and sequences that BLAST to GenBank FJ231390 (Supporting Information Fig. S1; Table S4) recovered two sister clades; one containing CBS 201.27 (GenBank FJ231396) from the Netherlands [one of the sequences utilised by Xu et al. (2010)] along with sequences from China, and the other containing GenBank EU152867 (Hu et al. 2010) and other sequences from China, as well as sequences from Egypt, Myanmar, Philippines and USA. Interestingly, the sequence representing CBS 385.63 was recovered as sister to the clade containing CBS 201.27. When BLASTed, the ITS sequence of CBS 385.63 shares only 91.19 % identity with CBS 201.27. However, BLAST results for LSU sequences of the same two CBS cultures found 97.99 % identity. Given the existence of two or more clades in our analysis, it is possible that there are multiple taxa represented among sequences in GenBank currently identified as Cr. hydrophila. We chose as epitype one of the sequences utilised by Xu et al. (2010), from the Netherlands (CBS 201.27) as it was recovered within a distinct clade with several other accessions (and thus more representative of this particular taxon), rather than CBS 385.63 which was recovered as a single branch. On the basis of the sequence information, S. hydrophilum clearly belongs in Rhizoctonia.