Newbie Needs Help to Identify Dendrobium Illness

[hmomof2]

Good morning,
I rescued this dendrobium appox 8 mos ago from a big box when they were getting rid of their neglected plants. It was unusual looking so it peaked my interest.
This new shoot is 6 months old. I keep it in a SW facing window and water it 1/ week and have it in a fine bark medium.
Over the past 3 days I have noticed first, the discoloration and today, the black spots. How should I deal with this in order to keep it from spreading to the rest of the stalk? I have never even seen it in bloom so would hate to lose it when it was doing well.

Thank you so much for your help and recommendations!!