I read your article on cactus rot and i believe thats whats going on. Details: we purchased this cactus off someone a couple months ago, repotted and out it in out home and now Sits in a northwestern large window. We made a mix of aprox 70% potting soil, 10% peat, 10%s and and some lava rock. Replanted and watered it with about 32oz of water the same week. The former owner had said it had been about 3 months from the last watering and usually gave it a gallon every other month. Here are pics from 2 months after we got it. Whats should i do? Cut the rot? Repot in more sandy rocky conditions? Ps. The ropes are not tight they are just in case until its roots are stable again.
The Cactus likely did not need repotting as they can live in the same pot for years.
They prefer the roots to be in a smaller compact area.
Too large of a pot can easily lead to overwatering and then root rot.
The soil that you plant your cactus in should be very specific to there nutrition and water needs.
A premix specifically made for cactus can be used or this link will help.
Unfortunately when the cactus begins to display the soft and sunken areas on the plant, a tell tale sign of root rot, it may be too late to repot and change the soil mix.