I have had this particular cactus for around two years now without any problems. After recently returning from holiday, I noticed some light brown discoloration on one of the stems. I cannot see any darker discoloration near the root, so I suspect it could be temperature damage or sunburn? (I live in the UK which can get below zero during winter.) Has anyone had any similar issues? Any advice would be appreciated!
Certified GKH Gardening Expert
Yes, this could be sunburn or potentially mild cold damage. Overwatering can also cause white patches to develop on cacti. In the future, you can protect cacti from intense sun or cold by covering them or providing some shade. If you move one from indoors to outdoors, do it gradually so they have time to adjust to the brighter light.
It looks like the cactus should recover, but if any parts start to appear rotten or mushy, you'll have to cut them out so the rot doesn't spread: