Hi, I was wondering why is my orchid, Oncidium Jairak Fragrance, developing black spots and markings on the pseudobulbs?
The substrate is sphagnum moss with perlite mixed in. I allow the substrate to dry out a bit before watering and letting the water drain out. I also miss the orchid lightly every morning, and they sit under a small grow light that’s set to turn on for approximately 8 hours increasing based on seasonal day lengths.
Thank-you for contacting Toronto Master Gardeners with your concern about your orchid.
It sounds like the care you are providing your Oncidium Jairak Fragrance orchid is appropriate — bright indirect light, some additional light humidity, good drainage, no excessive watering.
Without a photograph it is difficult to be certain of the nature of the black spots you are seeing.
Many Orchids of this variety regularly develop black spots on their leaves & pseudobulbs which may not necessarily indicate a problem or disease. If the spots progress to an actual rot, then this is another matter. If you find the spots unsightly, then cutting off the damaged leaf or pseudobulb may be the way to go. Doing this may also help prevent further spread of any rot.
A more extensive discussion about Black Rot in oncidiums is provided by this article by the American Orchid Society .
Here is an entry from the RHS website https://www.rhs.org.uk/plants/387346/oncidium-jairak-fragrance-gx/details with general information about your particular variety of orchid.
Good luck in determining the precise problem and resolving it.