Cymbidium bulb rot and yellowing leaves in Semi-Hydro.

[Ray]

I think that "crown rot" is a bit of a misnomer in this case, as that usually refers to rots that occur when standing water is left sitting in the crown of a plant, most often phalaenopsis.

As your situation is apparently a rhizome-based infection, I would guess that to be a fungal rot - phytophthora or pythium, maybe. They thrive in high temperatures and humidity. A systemic fungicide is called for. I think Cleary's 3336 WP is one of the best, but don't know how readily you can find it. I have no experience with Phyton 27 (copper sulfate) on cymbidiums, but they are pretty hardy, so I doubt there would be issues. Cym Ladye can probably advise better.