Q. My Zygopetalum leaves are turning yellow, then brown. What can I do?
Thank you for getting in touch Jack. I'm sorry to hear about the problems you are having with your orchids.
It sounds as though your zygopetalums may be experiencing too much moisture in both their growing medium and their environment. These orchids are epiphytes and very intolerant of a planting mixture that does not drain freely. Whereas the moss in your planting mix would be ideal for many orchids, zygopetalums are better in a fast draining mix such as bark and perlite. For the same reason, they also do not like a traditional, deep orchid pot and prefer something far shallower.
Your day temperatures sound fine though a drop of ten degrees or so at night is ideal. The humidity is running a little high for these mid-temperature range orchids. Around 40% is better and higher than that will start to damage the leaves.
Finally, check that their location is giving them the classic bright but indirect light they prefer. All sorts of things like tree shade, curtains, nearby buildings, can affect the degree of light available regardless of the exposure, but direct southern and western light may be too intense.
Let us know if we can offer further help.
With best regards,