Saludos everybody
What should I do with this little fella?
I´ve been taking care of this lovely cactus for years. But due to a recent change of addres I had to move their ususal set in wich they were so happy. and perhaps a little overwatering has lead to this blackening of the cactus top.
What actions are to take now?

Im no cact expert, never seen this.. But what I have done with different kind of rots/infections in cact is simply cut the problematic part off, have a clean cut that just heals back up, and a pup will eventually grow. These cact are very sturdy.

I'd do that specially if the black starts spreading. But maybe someone knows specifically about this problem and how to fix it without having to cut, lets wait for other people's answers

you got to cut that thing and water it less

Is it also soft, rot will be accompanied by soft tissue....like poke your finger threw soft...in extreme cases anyways. Plus I have never seen rot start at top, always the roots...so if rest of plant is okay probably not rot.

Now if the black is not soft at all...it's a new place...new pathogens. I would say the stress of moving opened it up to a fungal infection.

Or it got too much sun and got burnt, but I doubt that, but slight possibly.

Answer is to cut it of, but make sure to cut at least one of the points where the thorns poke out. Like straight threw the middle or right at the top, and this is where your regrowth will emerge. Now I hAve one that is putting of on the side ones too, but that triggers the growth better.

If it is a fungus just keep an eye on it. I think in most case you will not have to treat it. Good thing this didn't happen to your loph, although I think if it is fungus it could possibly block off the fungus to that section of plant and just have a big black spot.

But since it's not your loph, cut that shit off!!!! Haha

I have seen this!

Someone left a wash-cloth under the pot, which had holes in the bottom, it's a mold or fungus on your roots, re-plant your cacti in a clean pot and fresh soil and it will heal.

In this picture you can see where the "black mush" healed, and where a large arm grew out.

In my case, changing the soil, and re-potting in a clean pot did the trick.

-eg

In the picture you can see that the black mush hardened.

This cactus had the black mush issue about 3 years ago, when it happened this cactus had no arms, no, as you can see in the picture in this post, it healed and has grown a 4.5 foot long arm.

My guess would be your soil is not well draining enough, maybe a little too much water, or something in the soil is rotting, but I can bet there is something moldy or a fungus on the roots...



-eg

Gracias for the help everybody
I´m going to cut the sick portion.
Would this be a good moment to perform a grafting over the cutting?
enteogenic-gnosis I double checked the soil and the bottom of the pot looking for any visible mold or fungic attack but didn´t found any sign yet. Should I remove the whole soil to check roots?

You can cut it off or you can leave it. If you put it back in good sun with less water the new layers will most likely grow nice and green.

Either way works, but if you cut if off it your cacti could very well grow pups at that point as well. which is not a bad thing.

Peace.

In my case this is what happened:

A wash-cloth was left underneath the pot the cactus was, this cactus pot had holes in the bottom of it, and water had drained through the soil out of the pot and on to the washcloth beneath it. Everything looked fine, other than the top of the cactus turned into black mush, and the washcloth beneath the pot had turned black due to some fungi or mold.

I did not remove the damaged top (though I probably should have), I simply removed the cactus and it's roots from the old pot and old soil, and placed it in a new pot with new, clean, soil.

I purchased a new bag of cactus and succulent soil, I added some sand, volcanic stones and perlite to to improve drainage, and placed it into a new and clean pot, I then replanted my cactus into this clean pot with clean soil, and it healed.

It took about a month to fully heal, and has been doing amazing ever since.

It's important that you have very well draining soil, and that you take steps to ensure molds and fungi do not attack your roots. I know it seems counterintuitive to see a problem at the top of the cactus and have its source be the roots, but in my case that's is what happened.

You can see from the pictures that the black mush simply hardened and the stalk began putting out pups shortly after it healed.

It looks like exactly what had happened to my cacti, all I can say is that in my situation, clean soil in a new clean pot did the trick, it healed nicely and has not had any issues since.

-eg

When you buy the cactus soil mix, its a good idea to run it through a screen to catch all the hunks of wood and what not that use to bulk up the soil. Its no good. Mostly because it holds moisture.

Saludos everybody
So I finally cutted the afected portion and decided to perform a grafting.
this is how it looks now.

So far all the soil mixes I have seen labelled ´cactus soil´ have been TOTALLY unsuitable for growing Mexican desert cacti, being full of acidic peat and ´forest product´ - read sawdust. Many desert cacti grow in alkali limestone soil, often with huge chunks of the stuff.



My own mix for this sort of cacti is gritty and well draining, with little organic material - there is a good % of lime which the plants do well in.