About a week ago, an anonymous commenter insisted that my oncidium root problems were not due to overwatering or wrong media, but rather to a fungal infection. After some reflection, I realized that she may be right. In my attempts to nurture these orchids, I had overlooked the preventative and/or curative measure of treating them with an antifungal. So I went and ordered some systemic fungicide.
After over a year, my two oncidiums have failed to produce sufficient root growth. Both my Oncostele Pacific Perspective, and my oncidium noid would grow new roots that failed to thrive. The oncostele has lost two pseudobulbs to rot over the summer. In addition, my Psychopsis Mariposa 'Mountain' had suffered from root rot when it arrived to me. So while the rest of my orchids seem to be doing well, these three are struggling.
Since I use the same bucket of water when watering all my orchids, I felt like I should treat my entire orchid collection or else I'd just transfer any fungal rots back around the next time I watered. I used the dosing instructions from here for "thiophanate methyl" and soaked all my orchids. I wet the leaves of any orchids that had leaf spotting, but didn't bother with wetting the foliage of my healthy looking orchids. I also let the sick ones soak for about twice as long (30 min) as the healthy ones (10-15 min).
Starts of new roots and new growth on Psychopsis Mariposa 'Mountain' after losing all its roots to rot
I repotted the oncidium noid and psychopsis mariposa into fresh dry sphagnum moss after the treatment. The rest of the orchids, I simply soaked in the fungicide treated water.
I hope this helps.
(P.S. Thanks Anna for bringing this to my attention)