It looks like some sort of fungal rot, perhaps brought buy the Stink Bugs that I also see in the photos.

Hi there - thanks for your thoughts. Probably is some sort of an infection, I agree. I have not seen any stink bugs however. I will keep my eye out. thanks again

How does the rest of the plant look? notice any leaves wilting or stunted growth?

You know, strangely, the plant beyond the rotten part on the stem looks fine for a time. Not sure how long. However, eventually the leaves wilt and it dies. thanks Peggy

I don't see stinkbugs in the photos - what am I missing?

Stinkbugs of course.....

Ha!

OK, I went back and looked again. I still see what could be a stinkbug-type hanging on to the stem, upside-down, in the far, far right middle of the second photo - if you click it larger. There is also a little beetle-type just to the left and above it. AND, there is something yellow right in the middle of the photo. What are they?

The yellowish stuff looks like dead plant matter to me. I see what you're referring to on the far right, but it's impossible to make out clearly. I know those kinds of bugs will move away quickly to hide, so you could be correct.