My plant's one stem has gone dark and greyish from the bottom up. The afflicted stem is still very sturdy. The leaves on the stem fell off but they did not dry up or go grey first. I haven't watered for a week now, since the plant started showing symptoms, and the drying process has slowed. I assume I will have to remove the afflicted part of the plant, but can I save the unaffected top half? What is the best way to proceed?