Thank you for your reply. But can you explain me how a pot could be too big? In nature they don't have a limited area.

Regarding my Pilosocereus azureus, are they too big for the pot or is the pot too big?

Hi,

which species? I got these guys from a friend so no tags on them.

Here there are two pic of my cactus with zoom on the rot.

I live in Denmark. I sprayed once or twice since October/November. So I really don't think it's an over-watering problem.
How should I check the roots? They are under the soil and the plant seems well anchored, ie it doesn't swing when i touch it.

Hi, I live in Denmark. I keep my Pilosocereus Azureus inside the apartment during the winter months. I bought it in 2020 but it is just in the last 3-4 months that it seems suffering. The head is turning bluish/violet. The big brother planted in the same pot is healthy. What is it happening? What sh...