Dendrobium dying????

I have had this dendrobium plant for a couple years now. It has been one of my very best bloomers with magnificent flowers in the past and one of my very favorite orchids. However, one of the main stems looks like it is dying and maybe that could mean the whole plant?

The seller did give it a name (Den. Sugar 156) but it is an unregistered orchid. The first two pictures are of the dendrobium today. You can see the black stem. It was quite a bit taller with many leaves, but now the leaves are turning yellow and brown one by one and falling off the black stem one by one. I have uploaded my individual plant file of Den. Sugar 156 so that you can see what the plant looked like previously

So, can anyone tell me what has happened to the plant? And what should I do? Remove the bad or diseased stem? Or am I going to lose the entire plant? The other stems and leaves appear healthy with no problems. Thanks for your help.

Orchids Forum did not allow me to upload pictures the first time. I was able to enter the pictures doing an edit of my thread.

 

Thanks Ray. I believe it has a disease, so I will cut it like you suggest.