Help, I've just potted this guy from a cutting a few weeks ago (the cutting had calloused).
Over the last few weeks I've notice these black spots coming up, worse in the last week.
I have tried spraying with a fungicide Metconazole. Thanks for your help.
I live in Southern part of Western Australia, the cactus is outside under put patio with arvo sun.

Do you know your humidity locally?

I think the entire crown may have died. It will eventually have to put out pups. Spraying it with fungicide should help.

The fungicide should help, but you want to keep a close eye on the crown, if that black spot developing there keeps spreading you are probably looking at a deeply seated rot situation and you may have to behead to check how far it has gone.

That is problematic as it is doubtful the plant has many roots yet.

If beyond spraying it you have been watering the plant: stop, without roots all that watering will just form the cause for the rot. I guess you could take something to grab the plant with and gently tug on it to see if any rooting has taken place.

Based on what I think the plant is there is plenty of hope that this plant will be able to pull through, but it may require some more surgery depending on how the grown spot develops (the other spots so far look ugly but not too concerning to me, though together with the crown rot they could be indicative of a more major rot situation and at that point the plant is probably SOL).

Good luck and keep us posted!

Thank you.
We aren't in a very humid climate, at the very bottom of Australia.
We have had some very hot days, but also thunderstorms lately.

I doubt it has any roots yet as it's only been in the soil 2 or 3 weeks. Should I pull it out and let it dry out again?
It was so healthy looking when I potted it 😭

I also have some black spots in the top of this cactus. But they seem to be on the top of the skin and not under.

I would not lift it out of the soil just give it a gentle tug using some kind of grabbing tool. You should no immediately if there is any resistance if there is it has roots, if not at all, then it is still very much rooting in.

Those spots look like some kind of fungal - so moisture related - outbreak, which can happen to plants like this when they are not used to much humidity at all or it is really hot and all of a sudden humid...

Spraying with antifungals will help, but other than that I'd keep an eye on it but would not be too concerned. The plant in the OP you want to keep a sharp eye on the tip if it gets worse you will want to lift it out of the soil and probably decapitate it, but for now it may just pull through. It will probably not grow from that tip, but may branch there.

Also, when rooting cactus cuttings, especially when they aren't opuntia, don't water for at least 2-3 months. Ideally, wait until it looks like it is screaming for water and shriveling before you put any water in the pot and protect it from rain.

I would not lift or tug to check for rooting but I would cut the top off (without pulling it from the pot). I would want to stop that rot. My fear is you will find the pot is all the way down the center and breaking out at the areoles.

If you do end up taking it out of the pot and doing more surgery, I would place it on its side so there are more spots to root.

UPDATE

I Have been spraying the original cactus with fungicide. But it seems to be getting worse. The rot on the crown is bigger and there is alot more rot along the stem. Pictures below. Is it time to cut the crown off? Also do I need to remove the black from the sides? There won't be much cactus left.

I have also noticed the fungus is on my other cactus now. Pictures below. That all have it in the crown
I did quarantine the original cactus.

Would it help if the cactus were in full sun?
At the moment they are under a patio which gets direct afternoon sun.

Thanks

Cut the crown off! Do it as soon as possible.
When you cut it off, cut until there is HEALTHY tissue. Tissue that does not have any brown or black in it. Maybe even dust the cut wound with cinnamon to prevent further rotting.

Thanks.
Do I need to cut the rot out of the sides?

I would cut it off too. Get a good sharp knife. Make sure the sterilize the knife between cuts, just to be safe.

Yep, cut out all the rot until you only have healthy tissue left.

Make sure the tissue is healthy. You don't want to under do it.

I have just started to cut to black spots out.. not sure there will be much cactus left 😭

Is the rot inside or just normal tissue?

Thanks

That is not normal. It's not looking great...
I wish you good luck. If you find some healthy tissue, it might survive.