Dark spots on top of a healthy cactus ?

[1agree]

Hello, I bought this cactus maybe half a year ago, and it has seemed to be very healthy and growing, but few days ago I noticed this small black spot on top of it.
I tried to remove it (precisely cutting it off with a knife), but tomorrow it just returned and seemed to spread around a little.

To make it even stranger, as you can see on the picture, the column which has it is actually the biggest and tallest one, so I guess it should have the less chance of being infected ?

Please tell me its just some kind of a sunburn...

[Tony]

The plants are not getting nearly enough sun and that is why they are growing thinner at the top.
The dark spots are probably because the plants growing tip is starting to die off.
The two taller ones will never look right so I would chop them at the point at which they start getting thin and let them grow new healthier stems after you gradually adapt them all to stronger light.

[1agree]

seriously ?

In the meantime, the black area spread a little bit more, and I think it is starting to appear on the other one too (something very light brownish on top atm)

...My apartment is on the north side of the building, and that plant is on the spot with maybe the most sun that can possibly get (window is immediately up right on the pictures - where the light is coming from). I could only place him somehow, maybe somewhere more closer to the floor, so all the columns get the same amount of light..

The deviated columns actually looked kind of cool to me, and I thought its normal for plants to lean towards the light, but I guess its better to have a regular grow.

I'm just happy its not some kind of disease or rot (but I guess that is not certain yet).

Tomorrow I'm gonna try and cut those inappropriate grows, and find some place with better light..

Thanks for the help

[Saxicola]

It is normal for a plant to grow towards the light, especially indoors where the source of the light is very concentrated in one spot. If that were all it was it would be no big deal. But if a cactus suddenly makes much thinner stems that are a lighter green color it means it is getting way to little light. Do the best you can do but it may be that there simply isn't anywhere in your apartment that the plant can get enough light (north facing windows are the worst direction for light... assuming you aren't in Australia!). Do you have a balcony or anywhere the plant can be outside, at least for the summer?

[1agree]

this is a fresh picture of a brown spot on the other one - are you positive it's not some kind of rot or disease ?

btw, current place is closest thing to a balcony... actually it was a balcony once but I closed it and now there are other plants too besides this cactus (2 cactus also which are just fine)

[1agree]

new development :

[luddhus]

It might be rot, sometimes it starts from the top. I would cut again, about 3 cm from the top and using a sterilized knife.

[1agree]

Hm, I just tried to remove a plant from a double pot and this is how the original pot looks like at the bottom. I am newbie and so I do not know is it good that the roots are coming out or is the repotting necessary ? (image is a bit blurry but thats all root you can see at the bottom coming out of holes, with one longer part).
Btw the 2nd (outer) pot does not have holes.... maybe that was the problem ?

Also, the black spots begin to appear more like some kind of sunburns (looking dark and scorched)

[hoteidoc]

Roots out the bottom a (positive) sure sign repotting needed.

[Tony]

Roots out the bottom is not the cause of the problem up top however.
Chop off the infected parts before you possibly lose the whole plant to rot, re pot in a larger pot with good drainage in a fast draining mix and gradually adapt them to stronger light.