That is how it was with black rot for me. Is it nutritional or do you think it is virus?

With dendrobium "mystery ring", I think it is definitely a disease not a nutritional because I treat all my orchids the same way but only certain plants get the rings. only on dendrobiums. funny thing is the common varieties do not get sick but only the rare ones that I really like. so annoying!
Also, it is supposedly rather common but I cannot find any information about the ring disease of dendrobiums.

Black rot is not viral. It is fungal most likely.
I believe the most common black rot occurs when temperature and the humidity are high.
I have never gotten black rot on cattleyas yet.

My problems with catts are one, they usually do not bloom for me, two, certain plants develop weird marks on the leaves but grow just fine.

My black rot was caused by either calcium deficiency or the catts being planted too low. I repotted the rest and give extra calcium so have not had a problem since with that.
Interesting about the dendrobiums. I hope you figure out what is causing the markings (and that it isn't virus).