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...well... Two things happened.

Firstly I stumbled upon http://www.cactusinhabitat.org/index.ph ... cie&id=118. There are loads of obviously healthy Stesonia pictured, usually with drastic scars & patterns & with strange looking cork. This taught me something.

Secondly I am quite slow. Today I looked again at my plant, thinking of the wild ones, and was astonished: Everything seems to heal & to heal fastly!

So I admit, I will give the plant another week before doing *anything*, especially anything drastic. Then let's see if "the healthy plant get's away with it".

Thanks for your help & concern. Keep you updated.
N.

I concur with zpeckler. Looks like sunburn and over watering.

...let's hope that it really is sunburn & too much rain. That is curable.
A fungus would be fatal...
N.

nachtkrabb wrote: ↑Fri Sep 22, 2023 9:57 pm ...let's hope that it really is sunburn & too much rain. That is curable.
A fungus would be fetal...

Hmmm, I think you mean "fatal".

I noticed something in one of your photos:

It appears that you took the soil out of the Stetsonia's old pot and surrounded it with new mix when you repotted. If that's what you did, unpot the plant, clean the old soil out of the roots as thoroughly as possible, let them dry out, then repot in the new mix. IMO years of old soil (and too much of it) got the better of the Stetsonia's roots, and unfortunately the consequences seem to be getting worse. This is what happens when two fundamentally incompatible mixes are being used in the same pot. Your new mix sounds pretty good, so if you get the Stetsonia's roots going only in that mix next spring, it should recover nicely.

Steve Johnson wrote: ↑Sat Sep 23, 2023 4:59 am It appears that you took the soil out of the Stetsonia's old pot and surrounded it with new mix when you repotted. If that's what you did, unpot the plant, clean the old soil out of the roots as thoroughly as possible, let them dry out, then repot in the new mix. IMO years of old soil (and too much of it) got the better of the Stetsonia's roots, and unfortunately the consequences seem to be getting worse. This is what happens when two fundamentally incompatible mixes are being used in the same pot. Your new mix sounds pretty good, so if you get the Stetsonia's roots going only in that mix next spring, it should recover nicely.

This sounds like a very interesting idea & strategy without cutting up the plant. Of course I had tried to get off most of the old mix when repotting, but it really was too much for me single handed. I am quite ill yet.
But, please: How come my mixes should be incompatible? What happens here?
Besides: This is not the first plant with a very similar switch where some ld soil remained.
Thanks
Nachtkrabb

nachtkrabb wrote: ↑Sun Sep 24, 2023 8:38 amBut, please: How come my mixes should be incompatible? What happens here?

Differences in moisture retention -- poorly-aerated soil takes longer to dry out, and roots won't grow as well as they would in a free-draining mix. If there's too much poorly-aerated soil left in the root ball after repotting, the roots eventually suffer.

nachtkrabb wrote: ↑Sun Sep 24, 2023 8:38 am But, please: How come my mixes should be incompatible? What happens here?

I second what Steve said, the different mixes have different moisture retention. If you repot a cactus into a new, presumably better draining growing medium but don't remove the ball of old medium from around the roots you really haven't improved anything. The old, poorly-draining medium is still sitting right around the base of the cactus holding onto water and promoting rot just like if you never re-potted it. It's a long-established cactus cultivation "best practice" to completely clean the old medium off as much as you can without causing excessive damage to the roots when re-potting.

Here are some relevant forum topics:
A Beginner's Guide To Repotting Newly-Purchased Cacti
(Note: it's no longer recommended to put the layer of rocks at the bottom. It doesn't improve drainage and just creates a "perched water table.")

Garden Centre Plants In Peat

Free The Roots!

Guys, thank you, I understand a sentence about different forms of drainage. After reading the above I expected at the very least some strange chemical reaction producing prussian acid or something!
But you needn't give me a beginner's kit, I am growing cacti pretty successfully for a longer time than many people live which are posting here today.
Actually, some of my succulents & cacti might live longer at my house than many people live which are posting here today.

I just checked at my repotting notes: Of course I had gotten off as much of the old soil as possible (what do you think?). Only the old soil really was no soil anymore but just a compact ball of roots. If I had wanted to remove more than I had, I had had to cut off the whole of the roots in one go. Strangely, this I didn't consider as a possibility.

Today in the afternoon I had rechecked the plant. It is continuing to heal with an unexpected velocity, after it had done nothing for quite long. The two specks on the arms look almost harmless now: What shocked me were those discolourings in the surroundings of the marks, as if the bad parts were growing to the inside, or growing from the inside -- the way I would expect some fungi to show. (I am not sure if I ever had a cactus with fungi but I have seen pictures & scenes in documentaries.)
Also the big gap is callousing over and healing, so that can't be that bad, too.
After all, today I tend to "a really badly scorched sunburn".

While I am writing this, it is pitch black night, as far as possible in a city. As in such cases photos taken with flash are useless, I can't add a new photo now, but I will.

Have a good time, take care
Nachtkrabb