Three of my dendrobium hybrid suffered this same thing last year.
I searched but couldn't find out what it is.
Whatever it is, those dark spots or rings will spread all over and cause the leaves yellow and drop.
I sprayed with some chemical but it doesn't seem to work.
I removed affected leaves, then those rings would come back on healthy looking leaves. very persistent.
so I ended up dumping all three in fear of losing my other dendrobiums or other orchids. I have many dendrobiums, so I had to let go of bad ones.
Since then, no such outbreak and I really hope I don't have to face this mytery ring disease again whatever it is.
I would put the affected plants away away from everyone else. removed the affect leaves and see if they come back on the healthy leaves.
If they do, then it's best to discard the plants.
Last edited by NYCorchidman; 01-27-2013 at 10:56 PM..