help, black spot on one of my babies

[hegar]

Most likely, the other three cacti will not become infected. This seems to be a localized problem, with perhaps a weak pathogen having been able to enter at the initial injury at or near the areole.
A treatment of the other plants may not be necessary. Also, the treatment would depend on what kind of organism invaded your cactus.
Sulfur does work against fungi, but I am not sure about its efficacy against bacteria. Most of the time, a bacterial soft rot organism will take up residence in an injured plant. However, there are also a good number of fungal candidates that too would be able to kill your plant.
I believe, that just keeping your eye on the other three cacti in the pot and watching for any lesions or discolorations to appear would most likely be sufficient. If the organism were an effective (virulent) pathogen, your companion plants would most likely already have the same symptoms.

Harald

[daffe]

I used some suphur powder on the black spot. Its not easy to see on this picuture, but the black spot was shrinking and is now almsot gone.

[hegar]

Your plant does look better than it did September 10th, when you took the initial digital images. Even the chlorotic area has about disappeared and been replaced by a healthy green color. Your cactus seems to be on the mend. That is a great outcome, because cactus seedlings oftentimes do not survive any surgery, because there is not much that can be removed and by the time the problem is discovered, the whole plant has become infected. I had a cactus rot while I was in Germany, because it rained too much, killing the root system. It was a rainbow cactus with multiple stems of about 4 1/2 inches long and 2 inches in diameter. I noticed, that the tips still looked green, albeit a pale green. So I started to cut the stem toward the top, but even less than one inch from the apex there was still an internal discoloration visible in the center of the stem.
Well, that convinced me, that the plant was a goner. You can only try to root a stem segment, if it is healthy.

Harald

[greenknight]

There are some fungi that produce hard, black lesions that don't spread fast. Cacti generally are able to contain their spread by walling them off with a callus layer - unable to spread further, the fungus starves to death, but it produces spores before it dies. It may be that this infection is past the point of producing spores, but it would be safer to keep it separated from the others for quite a while ( I see you've moved it to its own pot - good idea). Keep the skin of the cactus dry, so any spores that may exist can't grow.

That will never go away completely, there'll always be a dead patch that will become more sunken as the plant grows around it. I've seen cacti that had those for many years, didn't seem to interfere with growth - just a bit unsightly.