Black Tip Rot on a Tall Columnar Cactus

A 10-year-old, 2+ foot columnar cactus suddenly blackened at the top 3 inches. Repotting didn't help. This is rot—cut it out. Use a sterilized knife, cut 1–2 inches below the black, re-sterilize and repeat until only healthy green/white tissue remains. Dust with fungicide or rooting powder. If roots were healthy at repotting and soil drains well, the plant should recover, though it will be disfigured. Columnar cacti may not branch after topping. Dispose of all rotted material—do not compost.