Spreading black spot on cactus

[krsimkins]

This cactus (not sure what type since I'm relatively new to cacti) was inherited from my in-laws.

A few weeks ago, we noticed one of its lower arms turning black and slightly soft. We amputated the arm, hoping that would prevent any disease from spreading to the rest of the cactus. We didn't put anything on the wound (I hadn't read about using cinnamon powder yet).

Then we noticed the blackness starting to spread from where the infected arm was amputated.

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As you can see, the blackness has continued to spread quite consistently from that point.

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Reading through these forums online, it seems like this could be an overwatering during dormancy, especially since our apartment has been bit chilly and hasn't gotten a huge amount of sunlight since it is currently winter where we are. However, the blackness or rot does seem to be spreading from the place the arm was amputated rather than the base of the cactus. So, maybe it could be an infection of some sort? It does look like there might be spider mite damage as well.

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Any insights and recommendations would be greatly appreciated! This cactus is almost 30 years also and it would be a shame to lose it.

[ElieEstephane]

Yes watering during winter could spell disaster. It's definitely a good idea to cut the afflicted areas again. However, this time use sterile equipment and cut until all you see is healthy flesh, sterilizing the knife after each cut and then apply fungicidal powder and avoid water. Some people use isopropyl alcohol or other stuff to sterilize. Personally, i like to cut above ground infected areas with a hot knife. It will be 99.9% sterile and in most cases seal the wound.
It's a cereus peruvianus monstrose and even if the bas rots, it can still be rooted. Don't throw it away.

[krsimkins]

Thank you! Is there a fungicidal powder that you would recommend?