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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 shall I do?

The soil is composed by pumice stone, cactus soil and coarse river sand (1/3 each) together with a bit of small lava rocks.

Check the roots. Could be a rot there. Watering during winter rest can be risky.

But if it's still dry for several months, everything is OK. Not knowing more about your location and conditions it is hard to advice you better.

Agree. Check the roots. Of course I didn't make that recommendation lightly because that's no mean feat with a plant that size, but anything that could kill the whole plant will have started with the roots.

Where are you in the world and what kind of conditions do you get there? Knowing that is helpful in giving specific advice.

zpeckler wrote: ↑Sun Mar 24, 2024 1:17 amWhere are you in the world and what kind of conditions do you get there? Knowing that is helpful in giving specific advice.

Here's your answer:

Cachi wrote: ↑Sat Mar 23, 2024 4:54 pm Hi,
I live in Denmark.

Cachi, please see this:

https://www.cactiguide.com/forum/viewtopic.php?t=43819

What Zac just said is the reason why I send that link to new members.

Do not water at all in the fall and winter even though you have your P. Azureus indoors. Also, desert cacti need good ventilation -- easy when they're outdoors, not so easy when they're being kept indoors. Stagnant air in the house (or in your case the apartment) sets up the conditions for fungal disease. Can't see the taller plant, but if you see black areas on the upper part of the shorter one, that's a fungal infection setting in. Post a close-up photo of this area:
Then we'll see what's going on. In the meantime, place a floor-length oscillating fan near the cacti, set it to the lowest setting, and get the air moving around them. Have you been watering them? If so, when was the last time?

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.

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

Cachi wrote: ↑Sun Mar 24, 2024 7:44 pmHere there are two pic of my cactus with zoom on the rot.

Yeah, looks like a fungal infection that started on the growth point -- exactly the problem caused by lack of good airflow in your apartment. Since you don't have an over-watering problem and both cacti in the pot are well-anchored, your best course of action is to cut out the infected area like this:

• Step 1. Sterilize a sharp knife with alcohol and make the cut at an angle as indicated above. (I'll explain the reason for the angled cut in a moment.) If the exposed tissue has a healthy green color, go to step 2. But if you see even a tiny bit of brown or black stuff, you'll need to continue cutting until you see nothing but healthy tissue. Sterilize the knife every time you cut.

• Step 2. Apply a layer of sulfur powder to the freshly-exposed tissue per this example:
  

  

  Echinocereus_pectinatus_'Coahuila'02062015_06.JPG (95.14 KiB) Viewed 2333 times

• Step 3. Let the exposed tissue dry out and form a callus. Gently brush the excess sulfur powder off.

Now the reason for the angled cut -- if you bring your pot outdoors for the spring and/or summer, this prevents the possibility of water pooling up on the cut surface as it would if you make a horizontal cut. Assuming that the roots are healthy (and I think they are), a new head should start growing on the cut in summer. If you get more than one, so much the better.

The most immediate concern is lack of good airflow in your apartment, so you really do need the floor-length oscillating fan I recommended. Are you fertilizing? If so, please provide me with a breakdown of its nutrient profile. From the guaranteed analysis of the General Hydroponics FloraMicro 5-0-1 and FloraBloom 0-5-4 liquid ferts I use, here's an example of the information I'm looking for:
Once I get your numbers, I'll let you know if your fertilizer is good for cacti. If not, we can discuss some better options you'll be able to find in Denmark.