Charlie that stem has been cut into to show the tell tale sign of fusarium visible inside the sick plant. Early Blight and the early stages of fusarium are so similar in appearance because the first yellowing leaves will sometimes have Early Blight as well. Early Blight is fairly easy to prevent or at least prevent it being a major problem.
Another sign of fusarium when it gets more advanced is the wilting of a whole stem. This wilting can happen with some yellowing leaves lower down or yellowing leaves way up the stem. Did you eventually have whole stems wilt?
I'm rather paranoid about fusarium since I have been battling its effects since I started gardening here many years ago. My first year of gardening tomatoes my county extention officer came out to look at my tomato problem and told me I would be unable to successfully grow tomatoes in my soil because of the severe fusarium problem I had. He was right in a way since my plants usually died from it within 3 months of being set out in the ground. Sometimes I would lose them all and sometimes a few would struggle on for a while longer. It wasn't until I started growing Celebrity and Big Beef that I was able to have a good crop of tomatoes most years. When I tired of the sameness of the hybrid tomatoes and went back to growing mostly heirlooms I coped by planting huge numbers and replanting as plants died right up until late August. Age and health problems have made that very difficult as the years have gone on and I started grafting last year and took the plunge by planting only grafted plants. I didn't have nearly as many plants to set out but they were far more productive and lived so much longer than usual. I still had to battle the usual foliage diseases like Early Blight, Septoria, Gray mold, Speck and or Spot. I also had a few cases of Tomato Spotted Wilt Virus but luckily last year I didn't have any Late Blight.
Just remember the old saying no matter how good your tomato plants are looking.
"Tomatoes are just looking for an excuse to die."
Bill