A comparative taxonomic study of Rhizoctonia sensu lato employing morphological, ultrastructural and molecular methods

A total of 41 strains, belonging to the anamorph form genus Rhizoctania sensu lato, the corresponding teleomorphs, and a number of unidentified North European orchid endophytes, were examined and assigned to the anamorph genera established by Moore (1987); Rhizoctonia sensu lato, Epulorhiza, Ceratorhiza and Moniliopsis. The septal pore ultrastructure, number of nuclei, and sclerotial morphology of these strains were studied. Restriction fragment length polymorphisms (RFLPs), in Southern hybridizations of restriction-digested total nuclear DNA and heterologous nuclear ribosomal DNA were used to distinguish between some of the selected strains at the generic level. Rhizoctonia sensu stricto comprises a well-delimited group, Epulorhiza should be restricted to Tulasnella anamorphs, since Sebacina strains have a distinctly different pore ultrastructure. The sclerotial morphology, pore ultrastucture and RFLP patterns of Ceratorhiza strains are indistinguishable from Moniliopsis strains, indicating a close relationship. Ceratorhiza can only be distinguished from Moniliopsis with respect to its nuclear number. Because Waitea strains have characteristic sclerotia and RFLP patterns, Moniliopsis is considered to be para- or polyphyletic Moniliopsis should be restricted to Thanatephorus anamorphs. Comparative studies of other related genera are required before new genera for the anamorphs of Sebacina and Waitea are erected. Rhizoctonia zeae was confirmed as the nomen anamorphosum of Waitea circinata through RFLP analysis of the type strain of R. zeae, CBS 384.34. The unidentified orchid endophytes could be referred to either Ceratorhiza or to Epulorhiza sensu stricto.