Tiny spots with circular yellow rings on dendrobium leaves (pictures included)
 

I bought a dendrobium phalaenopsis from a nursery maybe two-three months ago. It had snails so I had to repot, which probably stressed the orchid out. Anyway, it was the leaf apocalypse and all but four dropped within two weeks or so (on new and old growth). The remaining leaves looked pretty good but now I see this on the leaves and cannot seem to find any references which resemble it strongly. The dots are sunken, but not wet in appearance. My first thought was some sort of sun damage because we suddenly got that really bright, hot California sun this week but the yellow ring makes me think fungus or virus. The other large leave has yellowing, but less localized. Any ideas?

I suspect that those sunken spots are where water droplets stood and allowed an infection to take hold. Frankly, I'd not worry about it, as the plant will likely drop all of those leaves on the old cane anyway.

Anthracnose not real common but is common to these types of dendrobium hybrids. Usually the leaves will yellow, have circles or rings like that and fall off. Most often it will not cause harm to the plant, other than unsightly leaves and bare canes.
Happens in warm humid conditions with poor air movement. Good cultural practices, good air movement and limiting leaf wetness time should keep it controlled. No need to spray chemicals for this.