Association of orchids with endophytic fungi is essential in many cases for seed germination and subsequent plant development.

It has been observed that, in addition to phosphate and other minerals, the associated fungus provides the tiny seeds with a carbon source, as well as with vitamins and growth factors.

The balance between orchids and their endophytes in a mycorrhizal association is very delicate and often too much nutrition in in vitro cultures has an adverce effect on the seeds or seedlings, in many cases they are even killed by the fungus.

All endophytes associated with green (assimilating) orchids belong to the sub-division Basidiomycotina, class Hymenomycetes, form genus Rhizoctonia.

Identification techniques are based on number of nuclei per hyphal cell, hyphal anastomosis, cytomorphologyof hxphae and the morphology of cultures.

Isolates of Rhizoctonia possess  the following characteristics:

• branching near the distal septum of cells in young vegetative hyphae
• constriction of hyphae and formation of septa a short distance from the point of origin of hyphal branches.
• presence of dolipore septa
• absence of clamp connections, conidia, rhizomorphs and sklerotia differentiated into rind and medulla.
 

There is usually a clear distinction between the long straight main hyphae and short lateral hyphae that branch repeatedly and form chains of moniloid cells. Another charakteristic of many of these mycelia is, that in pure culture some hyphal tips coil into peloton-like structures almost they do within the orchid cells.

It is usually difficult to match sterile mycelia with corresponding reproductive mycelia (teleomorphs), but when the formation of reproductive organs is occasionally induced in vitro, they can be identified as teleomorpic taxa. In this way Rhizoctonia strains have been referred to a number of teleomorphic genera and among those associated with orchids several have been referred to species within Tulasnella, Sebacina, Ceratobasidium and Thanatephorus.

Tulasnella calospora and Ceratobasidium cornigerum  are the most common orchid endophytes. Strains referred to this fungi have establisched seedling mycorrhiza with several orchids from Europe and Australia in vitro.

Isolation of mycorrhizal fungi

Mycorhizal fungi are isolated from the roots or rarely from the protocorms of terrestrial orchids. Roots are first washed in water to remove most external debris and then cut in longitudinal sections. Under a microscope  individual hyphal coils (pelotons) are teased from the tissue with the aid of fine needles, placed into FIM-medium and subcultured until free from contamination.

References:

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BEARDMORE J, PEGG G.F   1981. A technique for the establishment of mycorrhizal infection in orchid tissue grown in aseptic culture. - New Phytol. 87:527 - 535.

BENYAGOUP M., JABAJI - HARE S. H., CHAMBERLAND H. & CHAREST P. M.  1996. Cytochemical and immunochemical investigation of the mycoparasitic interaction between Stachybotrys elegans and ist host Rhizoctonia solani (AG - 3).- Mycol.Res. 100(1): 79 - 86.

SNEH B., BURPEE L. & OGOSHI A. 1991. Identification of Rhizoctonia species - The American Phytopathological Society, St. Paul, Minnesota, USA.

TU C. C., KIMBROUGH J. W. 1973. A rapid staining technique for Rhizoctonia solani and related fungi. - Mycologia 65: 941 - 944

TU C. C., KIMBROUGH J. W. 1978. Systematics and Phylogeny of fungi in the Rhizoctonia Complex. - Bot. Gaz. 139 (4): 454 - 466.

VOIGT T. 1983.  Zu Problemen der Mykorrhiza bei Orchideen.- Mitt. Arb. Kr. Heim. Orchid. DDR 12: 9 - 31.

WARCUP J. H.  1985. Rhizanthella gardneri (Orchidaceae), its Rhizoctonia Endophyte and close Association with Melaleuca uncinata (Myrtaceae) in Western Australia.- New Phytol. 99: 273 - 280.

WARCUP J. H., TALBOT P. H. B.  1971 Perfect states of Rhizoctonias associated with orchids. I. - New Phytol. 70: 35 - 40.

WARCUP J. H., TALBOT P. H. B.  1980. Perfect states of Rhizoctonias associated with orchids. II. - New Phytol. 66: 631 - 641.

WARCUP J. H., TALBOT P. H. B. 1980. Perfect states of Rhizoctonias associated with orchids. III. - New Phytol. 86: 267 - 272.

WARCUP J.H.  1975. Factors affecting symbiotic germination of orchid seed - In SANDERS.F.E., MOSSE B.& TINKER P.B. 1975 Endomycorrhizas - Academic Press.

WARCUP J. H. 1971. Specificity of mycorrhizal Association in some Australian terrestrial orchids - New Phytol. 70: 41 - 46

ZELMER C. D., CURRAH R. S. 1995. Ceratorhiza pernacatena and Epulorhiza calendulina spp. nov.: mycorrhizal fungi of terrestrial orchids. - Can J. Bot. 73: 1981 - 1985.

ZELMER C. D., CURRAH R. S.  1995. Evidence for a fungal liaison between Corallarhiza trifida (Orchidaceae) and Pinus contorta (Pinaceae). - Can J. Bot. 73:862 - 866.