Green Gelato

I agree with Spinner Caribbean it's some type of fungal rot. I would locate a spot of healthy stem well below that point and cut off anything above that point and toss it. Look inside the stem of the cut make sure theres no brown or black coloring or cut a bit more off. You can use some ordinary grey modeling clay to plug the cut to keep bugs out. I would sterilze all tools in 1 part bleach 9 parts water. Let it soak for 5 to 10 minutes. Then rinse clean. Lastly you shoul do some defoliating try to open up the plant so more light and air can get at her. I've been seeing a lot of issue with this from grower in the northern areas where the cool nights are causing heavy morning dew to sit on the plants. The cool temps and moisture are perfect ingredients for fungal issues. I would keep an eye out for powdery mildew in the future. Hope this helps and you get some warmer weather.