Can you touch the black part how dose it feel??

Yes, it is dying. Everything black is infected with fungus. This was probably caused by overwatering, and the soil mix looks like mostly water-retentive peat. Fungus may have entered through a small nick in the roots, or stem.

The only way to save it now is to transect the stem above the fungus, dry the cut in air to callous, and place on top of DRY soil until roots form. That will take a long time, and it may still fail. For future succulent purchases, repot in gritty mix, and give no more water than it can use quickly.

This happens to all of us from time to time, so don't feel bad. You just need to start over, post a photo here for identification and advise on care.

Brad

as a gritty mix atheist i have to say that if watering is the problem,then not watering is the solution:-)

that said, your cactus is a goner-easier just to start over. living in a cool, damp place like toronto, the cactus might only need to be watered a couple of times a year and never in the winter.

dave,

It's not necessarily a goner at all - slice as Brad suggests, assuming the rot hasn't spread into all the tissue, and set aside - maybe a dip in some sulfur powder or Rootone would be helpful, too. It will need a warm place to root again, but that's eminently salvageable.

czarina,

It's a full sun plant, and it likes it hot. It will respond well to Toronto summers (I think it's a Pilosocereus, native mostly to Brazil). Follow Brad's advice, making sure the last cut into healthy tissue is made with a clean knife, and Happy Thanksgiving to you, too!

Dont throw this one away, there is still a chance! Take a clean sterile blade make a cut a bit above the black straight through making sure there is no rot or black marks inside the green tissue. If there is keeping cutting till its completely green. If you have sulfur powder or fungicide rooting hormone dust some of that on that surface and let it dry out. Honestly its getting late in the season, I would just keep it in a cool dark place then put it ontop of dry substrate come spring (something like pumice/lava rock) and no water till you see new root growth.

my point is, does she want to spend possibly months waiting for rooting and then quite possibly have it shrivel to nothing?

the key word is easier. unless it has some special meaning, dump it and do better with a new one.

and a cold, damp, dark toronto winter is probably not optimal for cactus rescue.

Really you guys beat me to it

I have a cactus with a problem like Czarina's but I've had it for 2 years. Originally, I had one I grew from seed. It was a about 1 foot tall. One winter it had signs of trouble near the base and then all of it turned in to mush. Then I saw a cactus at the store like the one I grew from seed and bought it. I hoped I could grow another large cactus. However, not long after I bought it I saw that same problem near the base and expected the cactus would turn to mush. That was two years ago.

I considered chopping off the top healthy part but didn't do it. I didn't do anything and it stayed that way and alive.

The white stuff at the base probably is mineral residue. One difference I noticed is mine has a clearer margin between the good tissue and bad. I can't remember if mine always had the clearer margin or if it was blurrier like the margin on Czarina's plant and became clearer later. In case someone is wondering: the cactus is in a mix of sand and coconut coir but the coir floats so there's a layer of unmixed choir on top. I watered it today.

Czarina, I'd try to save the plant just to see if I can do it. However, the irony is I might have saved mine, for now, by doing nothing.

davez7anv, I'm not a gritty mix atheist. Seems like a lot of people don't like the peat based soil mix the mass producing nurseries use. However, it works for the mass producing nurseries. Otherwise they wouldn't be using it.

Yes, but conditions of most purchasers of those plants are very different than the controlled light, temperature, and humidity environment of mass producing nurseries. Peat is cheap, and works well only when all the environmental conditions are tightly controlled. Otherwise, it is a hazard, not a boon. So your argument is weak.

If everyone grew young, small succulents in perfect greenhouses then maybe peat would be more popular as a mix component. But most don't, and the vast majority of specialty growers and collectors let their experience guide the decision not to use peat in mass quantities. The mixes folks use are quit variable, and what works for some may not be the best for everyone.

As for your discolored plant, that must be mostly corking, not the result of fungal infection because the stem still supports transpiration. Fungus invades the vascular system and shuts off water transport. Then it uses those channels to ascend the stem. stopping only when there is no more stem to infect.

There is nothing wrong with the do nothing and wait and see approach, but my experience tells me the plant in question is not going to make it. Of course I could be wrong ;-)

Brad

I didn't intend to agree completely with a pro-peat stance and make a strong argument for it. Actually, I prefer a gritty soil mix and I've complained to nurseries about using a peat-based mix. I just pointed out that using a peat-based mix works for the mass producing nurseries.

The dark lower part of the stem might be cork now but didn't seem like cork when I discovered it. It appeared suddenly and I didn't think corking just stops at a line. However, if the problem was a fungal infection then I don't know why that stopped at a line and why the lower vascular system wasn't destroyed. I might've tried using a systemic fungicide but I don't remember doing that. Some day I might try cutting off the top and rerooting it.

I guess I was the one arguing. I apologize for taking a stronger anti-peat stance than warranted. Everyone has to figure out the best cultural practices for their plants and environment, and for some that may include peat ;-)