Can someone explain to me what is happening to my Mandacaru plant ? I have had it for over two years now, and it has been growing rapidly during this time. However, ever since the plant accidentally fell over and a bump appeared on its stem, I have noticed a brown stain developing on its surface. About a year ago, I removed some of the brown spots using a clean knife, but the stain has continued to spread. I am really concerned about its health and wonder what steps I can take to help save it and restore its vitality.