Treating Black Rot on an Old Man Cactus (Cephalocereus senilis)

An old man cactus has a growing black, soft spot near the top—classic black rot. By the time it's visible externally, internal damage is usually more extensive. The only chance to save the plant is aggressive surgical removal.

Use a sterilized, sharp knife (wipe with alcohol between cuts). Excavate all black, discolored tissue until only clean, white/green tissue remains. Dust the wound with fungicide or rooting powder. The hole will not fill in but will callous over. If the rot is too deep, cut the entire top off below the infection, let it callous, and re-root as a cutting. Either way, repot in fresh soil and discard the old mix.

Success isn't guaranteed, but acting fast gives the best odds. Wear gloves when handling.