Originally Posted by Aaron Hicks

The reason they fail to thrive is that P668 is minimal medium that
is derived from Murashige and Skoog medium from back in 1963-1964,
originally designed for tobacco. However, M&S is way too strong, so it is
used at 1/2 strength for P668 (with a few modifications). Dropping this
back to 50% strength puts it at 1/4 strength M&S, which is more like what
is used for insectivorous plants- species that really can't tollerate any
concentration of nutrients.

In short, the medium is far too weak. There is also the
possibility (albeit remote) that the plantlets have developed some sort of
contamination that is either not visible or (even less likely) residing as
an endophyte.

Rather than perpetual subculture on something like 1/2 P668, try
something closer to 70% label strength O156 (PhytoTech) with ~8.5 grams of
agar for gelling. The difference between O156 and P668 is that the former
has banana powder + maltodextrin (a glucose polymer) added to it. You
raise the osmotic strength of the medium substantially, but the
concentration of sugars (from the banana and the maltodextrin) is what
they need. P668 is good for germination; O156 is good for replate.
However, O156 is way too concentrated, as it was optimized for
phalaenopsis hybrids by Ken Torres while he was at Sigma. Although it
works, it's too "hot" for most plants that don't grow as fast as phals, or
have roots smaller than phals.

Good luck.

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-AJHicks
Chandler, AZ