That looks like a bacterial infection. It also appears to have been introduced by breaking the stem while training. I have had this a couple times. No longer because I sterilize breaks with hydrogen peroxide and cinnamon when possible. I had one plant only look like that in one place and never spread. A couple others lost the branch. And one affected the plant. It is possible to spread. Do not touch your other plants with your hands or tools until they have been properly cleaned. If the spot grows I would separate your plants. If not just toss it. Better to lose one than all. P.S. Make sure your hands and tools are always clean before touching your plants.