Well, this is a puzzling problem. I too am experiencing some die back at times and cannot figure out what caused them. My older Echinocereus bonkerae had the tip die. It just hardened. The plant did produce callus tissue and the problem did not spread. The plant then developed two arms near the top. Right now one of my Peniocereus greggii cacti has its only stem drying up from the top. However, further down a new side shoot that looks healthy is forming.
I just hope, that the plant will only die back to that shoot and not continue on down. Another one of my plants, an Echinocereus fendleri produced a second stem, which at first looked healthy. Lately though it has turned very light greenish-yellow on the side, while the top is dark green.
All these problems I do attribute to environmental reasons, i.e. possible sun burn, frost damage, etc. Does your plant have the discoloration on the southwest side? Was it shaded at some point not too long ago near the point where the symptoms are appearing?
I highly doubt, that you are dealing with insect or disease damage.
When you removed the first (only?) stem, did you check below the blackened tip, if there was any indication below of perhaps the presence of a pathogen, for example a discoloration extending downward? Sometimes a cactus seems to be OK, until the inside of the stem turns into mush. Then it will start to lean heavily and collapse.
By the way, I too grow all my plants outdoors in sandy soil and also water them usually using a watering can once a week, unless we have experienced one of our rare rainfalls.
Harald
