Please help me save this guy!

[Brontosaurus]

This is a Lophocereus schottii v. mostruosa that I've been trying to revive. It's been heavily damaged by scale insects, and now it looks like it might be rotting on the top. I'm not giving up on Archie (I named it Archie)! Please tell me what I should do. Like me wherever you need to. Right now I'm keeping it under a growlight for several hours each day, and I'm thinking I can wash off the scale damage with a scrub brush (dish soap and water?). I want to cut out the dead part and repot it in a less fertile media. Is there anything else I should do? Are any of these things a bad idea?

[hegar]

First, I would check, if the top part is softer than the lower half of the cactus. If that is the case, you most likely do have a rot and you should cut off the discolored part with a sterilized knife. You can use either isopropyl alcohol or some other disinfectant like bleach (clorox).
I would cut the stem off at the point where it is constricted and the stem looks green. Make sure, that the tissue at the cut is not discolored. Leaving any discolored tissue will allow the rot to proceed and eventually kill the cactus. If the lower part is still salvageable, the cut should produce a tough skin (scar tissue) within a few days. The cactus will resume its growth, but most likely will sprout a slightly offset top from one of the areoles.

Harald

[Brontosaurus]

So I trimmed off the rotting part and scrubbed off all the scale damage. This is a picture of Archie from last night. I noticed this afternoon that there are some black spots forming where the cut was made. Is that healthy or is that more rot? Thanks for your help!

[greenknight]

Dark spots are bad, slice off more until it's clear if any discoloration. Dusting the cut with sulfur or other fungicide might help.

[hegar]

Greenknight is correct. The formation of a black discoloration is a sign, that the pathogen is still in the tissue. It is of utmost importance, that the knife blade is steril when the last cut is made. Otherwise you are transporting the pathogen right onto the open wound, re-infecting the cactus.
The cut end should form a light grey skin and the wound should then be considered closed to pathogens.
Sometimes it is very difficult to save a plant, because the disease (rot) has already spread too far. The plant tissue might look fine, but the discoloration is not yet visible, although the pathogen is present. Fortunately, within a day or two after you made your cut, you will see, if the wound is going to heal or if the disease is still present. So it is always a good idea to not stop right away when you no longer can see discolored plant tissue, but to make perhaps one more cut - with a sterilized knife - about 1/2 to 1 inch below the seemingly healthy tissue.

Harald

[Brontosaurus]

So here's some close up pictures of the cut end. As you can see, it's a little dark in places, but it looks to me like this is natural oxidization occurring in the organic tissue. However, I'm really not sure. I'm overly worried I'm going to do the wrong thing, so I wanted to make extra super sure that this is signs of rot before I cut the end off again. What do you think?

[greenknight]

I don't like the look of that at all. Better to cut too much than not enough, I recommend you cut some more.

I'd go to the next narrow point, and make a single straight cut. If you minimize the amount of cut surface it helps to minimize the amount of die-back.

Good luck.

[Brontosaurus]

Thank you so much for all your help!