My thought would be damage of some kind. The problem is that the central growth point appears damaged beyond repair, so the plant will have to generate a new growth point or points. If the visual damage is static and not growing, whatever caused the damage is probably in the past. If it is continuing, then I could well be wrong and it's a disease of some kind. Due the appearance in the center, I'd think the plant is vulnerable to disease as it's seen in the photo. If it were mine, I'd try to protect the center from getting too much moisture. I'd guess the plant will put out new growth if the stem is solid and healthy and the plant is getting sufficient light and air circulation. Just my opinion.
