A couple quick things to add.
I was reading a Japanese book about Cymbidium goeringii (
this one), and it mentioned the results of Korean University researcher. He isolated Tulasnella repent from some Cymbidium, and inoculated seedlings from flask and compared them to the control (no inoculation). After 1 year, the size of the inoculated seedlings were 1.1-1.5x larger. More significantly, rate of developing certain disease was < 1/5. Different variety of Cymbidium appear to have responded differently (1 variety got more benefits). I don't have the access to the original article, so I'm not sure about the statistical significance. But it seems to be a huge difference. He commercialized the product, which is called Oh-chid, but I haven't found any info about it. This product could infect fairly distant orchids (e.g. Phalaenopsis (Sedinera) japonica).
After looking around more info, the researcher is Kee Yoeup Paek (
link to his ResearchGate) from Chungbuk University (Korea). I found
a link to the abstract of their Chinese article about Oh-chid trial.
Another interesting thing is from a relatively recent article:
Sathiyadash, K. et al. 2012, Mycorrhizal association and morphology in orchids. Journal of Plant Interactions 7(3): 238-247.
They looked at 31 adult wild or cultivated orchids (all photosynthetic, some terrestrial, but majority is epiphytic). All orchids including epiphytes show strong association with fungi. The study included Acampae, Aerides, Bulbophyllum, Coelogyne, Cymbidium, Dendrobium, Epidendrum, Eulophia, Gastrochilus, Habenaria, Luisia, Malaxis, Oberonia, Polystachya, Rhyncostylis, Robiequetia, Satyrium, Sirhookeriana, Spathoglottis, Vanda, Vanilla. I'm guessing that non-Indian orchids are under cultivation (they didn't mention which one is from wild and which one is from cultivation). The infection rate doesn't seem to be drastically low in non-indian species, which is a bit surprising to me.
Another interesting thing is that infected region is patchy in roots of epiphytes. Here is the quote:
"All epiphytic species occurred in association with accumulated organic debris, mosses, and other plants. Aerial roots were colonized only when they were in contact with the substrate, while the roots which were not in contact were free from colonization."
It makes sense, but this might be a part of the reason why mounted and potted orchids may require a bit different fertigation scheme.