I have a tomato plant where the large majority of it looks great. However, one of the four large, separate major stem sections started to die out. Today I cut it away and found something I've never seen before.

There was some kind of black/gray growth inside. So I cut a cross section and the entire inside was filled with it It is fluffy and soft and similar to mold looking but reminds me more of clumps of loose dog hair. Here are a few more pics. Any help is appreciated.