How do I save my blue torch cactus?


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My blue torch cactus had gotten a black spot which I noticed started spreading so I decided to cut off a chunk around it to prevent it from killing the rest of the plant. I used a sterilized blade. But it looks like the black discoloration is coming back? (Don’t mind the brown-I had sprinkled cinnamon on the wound to help it heal and try to kill some of the bacteria as I don’t have sulfur powder for it.)

Do I need to just cut the entire top off flat? Or use some kind of treatment on it?

(I’ve included a picture of it before I cut the section out to give the best idea of what’s going on.)
 

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Cactus rot is hard to stop once it gets started. I would make a slanted flat cut at least six inches below the visible rot, even more would be better. Cactus rot travels within the stem a long ways before it is visible. Sometimes it becomes impossible to save any part of a rotting cactus. By the way, f you cut off a part that doesn't show any visible rot, keep that part of the stem dry and unplanted until late Spring. If it is not infected with rot, you can root it, and produce a new plant

Keep both rooted and unrooted pieces of your Pilosocereus pachycladus indoors in a dry, bright space over Winter. This cactus is a tropical species from Brazil and will not survive much cold. I do keep mine outdoors in Winter under a porch roof that protects it from rain and frost, and there it does survive nighttime air temperatures that briefly reach the high 20ºs F, but that is close to its limit.
 
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Update: here is what it looks like now. Don’t mind the little hole in the middle—I accidentally cut into it a bit there when I was slicing the top off :|

What are the brown spots? Is it more cactus rot? Sunburn? I’ve had it directly under a grow light.

The little section I also cut off between the top (which had rot) and the base (pictured) seems to be okay; I don’t see these brown spots on it.
 

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Those spots might be the rot, but I so hope they are not. Cactus rot does move up and down the interior of the cactus stem faster than the outside.Check the spots every day. If they stay dry and firm everything is probably fine, but if there is any hint of soft mush, cut again. Consider applying a little rubbing aclohol to the cut. It might stop a very superficial infection, though it won't do too much for deeper problems.
 

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