Diversity and distribution of Ceratobosidium and Thanatephorus: What orchid mycorrhizal fungi can tell us

Ceratobasidium and Thanatephorus include important plant pathogens, but they also include mycorrhizal fungi of orchids. The lifestyles, distribution and phylogenetic affinities of some of these fungi are unknown, especially in the tropics. Fungi were isolated from pelotons in orchid roots and identified by sequencing the nuclear ribosomal ITS region from pure cultures and direct amplifications from colonized root tissue. Several novel results were obtained: 1) Most orchid-associated fungi did not appear to belong to currently described species. 2) A single ITS type of a recently discovered Ceratobasidium clade was associated with the orchid Ionopsis utricularioides and unexpectedly widespread throughout Central America and the Caribbean. 3) Ceratobasidium was significantly more common in Vanilla roots on tree bark than in Vanilla roots in soil. This was surprising given that Ceratobasidium is rarely reported from tree bark. 4) Some strains of Thanatephorus showed extremely limited growth in culture media. Growth limitations of Thanatephorus strains have not been reported previously. The phylogeny of the Ceratobasidium/Thanatephorus group is still unresolved, and the addition of mycorrhizal members may help clarify relationships.