Ceratobasidium cornigerum
Ceratobasidium cornigerum is a species of fungus in the order Cantharellales. Basidiocarps (fruit bodies) are thin, spread on the substrate out like a film (effused) and web-like. An anamorphic state is frequently obtained when isolates are cultured. Ceratobasidium cornigerum is saprotrophic, but is also a facultative plant pathogen, causing a number of economically important crop diseases, and an orchid endomycorrhizal associate. The species is genetically diverse and is sometimes treated as a complex of closely related taxa. DNA research shows the species (or species complex) actually belongs within the genus Rhizoctonia. Taxonomy Corticium cornigerum was first described in 1922 by mycologist Hubert Bourdot, who found it growing in France on dead stems of Jerusalem artichoke. It was subsequently transferred to the genus Ceratobasidium by American mycologist Donald P. Rogers in 1935. Molecular research, based on cladistic analysis of DNA sequences, places Ceratobasidium cornigerum within the genus Rhizoctonia, but this taxonomic problem has yet to be resolved. Anastomosis groups (AGs) Ceratobasidium cornigerum is one of several species whose anamorphic states are sometimes referred to as "binucleate rhizoctonias". These binucleate rhizoctonias have been divided into genetically distinct "anastomosis groups" (AGs) based initially on hyphal anastomosis tests, subsequently supported by analyses of DNA sequences. At least six of these AGs (AG-A, AG-B(o), AG-C, AG-D, AG-P, and AG-Q) have been linked to Ceratobasidium cornigerum, which may therefore be considered as a variable species (comprising at least six genetically distinct populations) or as a complex of morphologically similar species. In the latter case, it is not clear which of these AGs (if any) should take the original name C. cornigerum. Synonyms or associated species The following taxa belong in the Ceratobasidium cornigerum complex and have been treated as synonyms or as closely related but independent species: Ceratobasidium ramicola = AG-A (also includes several invalidly published names including Rhizoctonia candida, R. endophytica, and R. fragariae). This group contains a range of crop pathogens and orchid associates. Ceratobasidium cereale = AG-D (also includes the dubious name Ceratobasidium gramineum). This group contains cereal and grass pathogens. Ceratobasidium ochroleucum (= Corticium stevensii), Ceratobasidium lantanae-camarae, Corticium pervagum, Corticium invisum, and AG-P are all tropical or subtropical, web-blight pathogens. Description The basidiocarps (fruit bodies) are effused, thin, and whitish. Microscopically they have colourless hyphae, 3 to 9 μm wide, without clamp connections. The basidia are ellipsoid to broadly club-shaped, 9 to 14 by 8 to 12 μm, bearing four sterigmata. The basidiospores are ellipsoid and broadly fusiform (spindle-shaped), measuring 6 to 11 by 4 to 6 μm. Pale brown sclerotia are sometimes produced, measuring 0.5 to 3 mm across. Habitat and distribution If treated as a single species, Ceratobasidium cornigerum is cosmopolitan and has been reported from Asia, Australia, Europe, North & South America. It occurs as a soil saprotroph, producing basidiocarps on dead stems and fallen litter, but is also a facultative plant pathogen causing disease of crops and turf grass. It can also grow as a "web blight" pathogen on living leaves of trees and shrubs, particularly in the