Hi
I think this is a Euphorbia ammak i rescued a while ago from a spot where it was getting sunburn. The old scabs seem ok and have been there a while but the new dark area at the tip is new.
I'm thinking I've watered it too much trying to give it a fresh start in a new spot.

My questions are:
Is it a Euphorbia ammak?
Is the brown spot at the tip a cause for concern?
What should i do about it?
Thanks!

It is a Euphorbia but I don't know about the ammak part.

Yes, the tip is rotting. I would cut it off. Use a sharp knife and sterilize it between cuts.

Yes, don't be afraid to take drastic action there. The stem will branch below the cut, most likely.

I don't know how thick the top dressing is, but it's generally best to avoid a thick layer of rocks on top of the soil, as they tend to block evaporation that way, trapping moisture in the soil. When in doubt, with a plant in crisis (or post-crisis, as the case may be), go light on the top dressing. If that's an unglazed clay pot then this whole issue is sort of moot because water should be able to leave perfectly well through the sides of the container. How often are you watering? What kind of light does that spot receive?

Unless the plant is coming out of deep shade, it should be able to take lots and lots of sun. When in doubt, provide a gradual stepwise accommodation to the light (over the course of weeks), but this is a tree in nature and it worships the sun. The more the better, provided you do not experience extreme heat, of course. My advice would be to let it recover from whatever was stressing it out, then get it back out into the sun. Starting with a little bit, and then a little bit more, and so on. This time of year (in the northern hemisphere) the risk of sunburn is pretty small unless a plant is coming out of deep shade or the indoors.

Thanks for the advice.
To answer the questions: pot is glazed, top dressing is an inch on top of weed block fabric. It gets direct morning sun thru early afternoon.
Is the rot mainly due to overwatering (black spots- but jot mushy) different than the scabs further down the trunk (sunburn)?

I've stopped the watering a couple of weeks ago and will remove the dressing.
Ok, so how much to trim?
Picture 1, line 1 or 2?



Second picture is the other side of the same trunk.



Should i trim as far down as 3?





Third picture shows the scabs further down, which i assume i can let alone despite being unsightly.





The last picture is a trunk I separated off of this plant when it was repotted last year. It's doing quite well despite the scabs!

Get rid of the top dressing and the weed guard, that's just holding in moisture.

We can't tell you where to cut but you will figure it out. Cut just below the black and look at the inside. If its rotten, clean your knife with alcohol and cut again. Keep cleaning and cutting until the inside of the stem is greenish-whitish with no signs of brown.

The plant in the last photo is awesome!

especially about the last plant!

Maybe start at 1.5 in the first photo, then see how the inside looks.

I don't know if this is significant, but when i started to remove the dressing, the pot was swarming with ants. Nice little colony in there!

The soil was damp but definitely not water logged

Ants in the soil may be there for different reasons. They may actually live there but more likely (in my experience) they are there for either food or water. Food coming from root mealies or some other insect they farm in there, water coming from regular irrigation (depending on how dry it is outside where you live). They aren't actually feeding on your plant (most likely) but their presence may be alerting you to the presence of something that is. Was there/is there a trail leading from the pot away to somewhere else?

Ants can be pretty effectively removed by drowning (submerge the whole pot in water, wiggle it around to get the air bubbles out, and wait an hour) but they can also come right back as soon as there's air in the soil again, so that can be a temporary measure. If you identify problem insects in there, they can usually be treated by drenching the soil with soapy water. Like watering to completion, then waiting a few minutes and watering again, then repeating. I use Safer brand insecticidal soap, which I can buy in a concentrate, but I think you can make your own from mild dish soap at 1-2% in water. Then the next time you water, a week or two later (whatever), use regular water and you will start to flush the soap from the soil. It should not hurt the plant if it's at a low concentration and temps are not in the danger zone. You can also treat with a systemic (eg. drench the soil with imidacloprid) but I like to start with soap before I bring out the big guns.

My house has been having an ant invasion this last week and I learned something important. My homemade insecticidal soap doesn't kill ants! Safer's Insecticidal Soap is instant death to ants. I'm disappointed.

PS: I mixed my homemade concoction really strong as I wasn't putting it on plants, just the floor.

Windex or any other kind of ammonia-based glass cleaner also works great to kill ants (on the floor).

I agree the last picture is awesome like Daisyl and Baja said.

The dastardly deed is done!
A lot tougher than i expected and even harder to clean the knife!

Looks good. I don't think we properly warned you about the sap (avoid touching fresh sap and especially getting it in your eyes, nose or mouth) but you might wash your hands a couple times afterwards just to be sure you're clean.

It looks good.

Oops! Baja is right, not enough warning labels on that job.

Thanks
I was aware of the sap. Just surprised how stick it was on the knife.
I'm thinking of attempting to root the cut off bit

Ten days (already?!) since the cut. How's it looking to you?



Happily, one of the other columns has a few leaves "blooming",
So perhaps the location/light is ok?

Its looking good.

Wow after six months what a difference! I thought I'd post an update
Same location but less water and no weed cloth under the rock dressing

Nice!