I will go Google Disa cornuta again, because I thought I read a post on a different orchid forum about someone growing this particular species from seed, and that person was succesful in germinating them. The seedlings, if my memory serves me right, were not plentiful. He/she (?) said as soon as the protocorms grew their first two leaves he/she immediately transferred them to soil.
Soil composition isn't known. But I think this person used the method of using cooked oatmeal put on top of a banana peel and smothered the seeds in the oatmeal. No innoculation of fungi needed.
Adult plants are better to work with, they're stronger, reproduce faster and will in time produce flowers to pollinate and make seed from.
I think one of the major problems (this is just a guess), you guys probably just took the tubers, but didn't take the soil it was growing in with it. I also think, it has something to do with severe root damage.
You guys down in South Africa try this instead...find Disa cornuta or any other terrestrial orchids you guys want to save when they are growing or in bloom. Create a community of people willing to pollinate them by hand with you. Wait for the seed pods to ripen. When you see the seed pods just ready to split (I recommend buying a jeweler's loupe of at least 10x to see this), start harvesting the seeds.
The more seed pods from different parent plants the better. Higher genetic diversity = higher chance of proper replication without the fear of genetic mutations (too much inbreeding bad).
While harvesting, label each seed pod with the plant's scientific name, the date of harvest, which parent plant they came from, where the parent plant was growing (the more specific the better), and the seed pod #.
For example if on one Disa cornuta plant there were six pods, separate each pod into their own container and mark Disa cornuta #1 (Port Elizabeth, my backyard), (April 14, 2008), seed pod #1. The second pod will go...Disa cornuta #1 (Port Elizabeth, my backyard), (April 15, 2008), seed pod #2.
When growing seeds, mark them the same way as above.
As a note, if you haven't seen orchid seeds before, they look like a fine yellow powder. If you grow lilies in your garden and you rub the anthers of the flowers with your fingers, the pollen that get stuck onto your fingers is what the orchid seeds will look like (to the naked eye).
Now for the adult plants...
Mark the specific area where the plants were growing and WAIT UNTIL THE PLANTS GO DORMANT, before digging them out of the ground. But dig the tubers out and keep the soil they were growing in. THEN pot them. Always keeping in mind how they were growing when they were in the wild.