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Most of the early descriptions of fungal isolates from orchid roots were based on morphological characteristics of the fungal colonies grown on agar medium in combination with structural characteristics of the hyphae. Based on this, fungi isolated from roots of a number of orchid species were grouped in the form-genus Rhizoctonia. Interestingly, several species of Rhizoctonia, known to be pathogens on a variety of plant hosts, are able to form symbiotic associations with orchids, at least in sterile culture experiments. Following intensive culturing of isolates, and the recognition of anamorphs (asexual stages) and teleomorphs (sexual stages), several fungal genera are now recognized. The identification of the fungal symbionts in orchid mycorrhizas has also been enhanced by the use of molecular methods.
Although the systematics of the fungi associated with orchids is still in progress, some facts are known. Autotrophic orchid species (those that have chlorophyllous seedlings and adults) associate with different fungal species compared to myco-heterotrophic species (those that remain achlorophyllous throughout their life cycle). Autotrophic terrestrial orchids form mycorrhizas with basidiomycete anamorph genera such as Ceratorhiza, Epulorhiza, and Moniliopsis (corresponding teleomorph genera are Ceratobasidium, Tulasnella and Sebacina, and Thanatephorus, respectively). Heterotrophic orchid genera such as Corallorhiza are dependent on fungal symbionts for their nutrition (hence myco-heterotrophs) and form mycorrhizal associations with several fungal genera, including species of Russula, Thelephora, and Tomentella. These myco-heterotrophic orchid genera are able to form mycorrhizas with fungal species that link them via ectomycorrhizal associations with adjacent trees or shrubs; they use these hyphal linkages between their roots and tree roots to gain photosynthates produced by the autotrophic host.
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