I have raised orchids for years I have an Coel. Orchideengarten Joachim it is grown under lights, fans and humidity of 60%. Just recently just one leaf have developed this on it, does not seem to be spreading, but I don’t know what it is. Do you have any ideas of what it might be.
Thank you for contacting the Toronto Master Gardeners with your question about your orchids.
You are obviously an orchid lover who takes great pride and care of them , and am sure you reap the rewards of their beautiful bounty.
I am a snowbird who has many orchids attached or hanging from various trees in my south Florida garden. With the less than ideal conditions, black or brown spots are not an uncommon sight. Down there I usually just cut off the affected leaves if the spots seem to be spreading in breadth.
In your more carefully controlled environment you do have to be more careful not to spread the bacterial or fungal spot which seems to be starting on the leaf specimen in your photo.
So in terms of control, you can treat by swabbing full strength hydrogen peroxide with a Q-tip on the select spots , (but only on the top of the leaf). For an older leaf especially, I would not hesitate to cut off the leaf with a sterilized scissors, to do so, wipe them down with a cotton ball soaked in rubbing alcohol. When making the cut, be sure trim about 1 cm. past the black spot. If the entire leaf is affected, cut off the whole leaf at the base.
Culturally, you may want to decrease the amount of water slightly, especially on the leaves in question, and perhaps create a bit more separation of they are in close juxtaposition or touching neighboring. As an extra precaution you may want to isolate any diseased plants plants for a week or two.
Wishing you continued enjoyment with your orchid.