Sick Cactus - turned black

[Bcm3r]

I bought this cactus a week ago. I only watered it one time and the soil was never too wet. I called the store and they told me it was because I put the cactus near window. Does this seem correct? I am worried I was sold a cactus which was diseased to begin with. I realize the cactus cannot be saved .

The black spread in a matter of one day and it is very hard to the touch. Any insight very much appreciated!

[adetheproducer]

That's pretty dead looking. If there is any green stem at the top you may want to cut it off and re-root. Keep cutting slices away until the cross section is green.

[Bcm3r]

There is a tiny bit of green stem, but I am pretty disappointed as I have only owned the cactus for a week and I am not sure how this happened!

[adetheproducer]

I would say either it was on its way out already or too much water and not enough light and ventilation. The soil looks very wet and heavy with organics it would really have needed a much looser and mineral based and much more light than you will get indoors if that is where you keep it.

[hegar]

I just put "my two cents worth" in on your other post. Oh my, that poor cactus on the left almost looks charred. I just hope, that you can salvage the other stem. Where did you keep the plant? Did you put it outside in the sun? I do live in a city with very bright and strong sunshine at a high altitude. I have killed a cactus in less than three days by placing it outside in the ground with my other cacti. Those two plants could not take the change from a greenhouse environment with shad cloths into our searing sun.
In short, it is quite possible to "fry" a cactus, especially if that plant has not been exposed to the full power of the sun. If the stem is hard, most likely there is no rot present. Is the stem black all around or only on one side?
I just looked at your image and it is unusual, that one stem is practically a goner, with the chlorotic areas extending to the top, while the other stem looks relatively healthy. What does the dying or dead part of the cactus look like in cross section? It seems to be more caved in than the good looking stem. No matter what it was, but the sick or dead stem was exposed to a situation or an agent, which caused it to go into decline rapidly.
You need to be playing detective, in order to figure out, why one stem looks OK, while the other one is in horrible shape.
In case this is a pathogen, the good stem should be placed in fresh soil, after having been examined for any problem of course.

Harald

[Bcm3r]

Harald - thank you for your helpful advice on both posts!

The good news Is i think I will be able to save about 6 inches of this plant. I have been doing a lot of research over the last 2 days and have narrowed it down to a few things :

The smaller cactus was replanted by the store owner to line them up in the pot. He may hve caused serious damage to the cactus.

The pot was sitting in the back of a store with no light at all and I put my pot in front of the window when i brought it home. The temps have ranged from 60-80 in my NYC apartment and the window faces north. The change in environment may have caused this.

Some sort of preexisting pathogen in the soil. When repotting, I noticed that there was a 3rd cactus at one point in that pot which had rotted away and only the stem was left. I hope I can save that 3rd cactus quickly!