garf, it reminds me of a bacterial wilt. The bacteria clog up the vascular bundles of the plant and cause a brown discoloration. I note that it shows up on only half of the perimeter; and that can indicate that the infection is coming from the roots associated with the vascular system on that side of the plant.
Whether it is the same (or opposite) side is often debated.
A long, slanting cut exposes more length of the conducting tissue and shows brown streaks. This tends to confirm the diagnosis.