What's wrong

Please has anyone go an idea of what's wrong with this cactus.
(Sorry if it should be in the cactus compartment, but it is more about the disease than the cactus)
Why is it going like this and what should I do about it.
Even though I have been collecting them for 50 yrs, I am still hopeless when it comes to things going wrong

 

@Michael Hewett Oh dear that does look nasty sorry i cant help as i have no experience of cacti but do hope someone on GC can aid this poorly plant

 

If it was anyone but you I would suggest that it has been overwatered but I know you better than that. So a few questions;
Did you check for mealy bug or any larvae in the growing medium?
How long have you had it? It could have started before you got it if you bought it recently (ish).
Where is it kept? Cold draughts, exposure to cold or condensation dripping on it etc.
Is it exposed to magnified sunlight?

Just a few thoughts really.

 

Is that a cutting?

It looks vaguely like Stenocereus, which if it is, likes a bit of warmth and hates winter wet, condensation etc. as longk suggests.

I've got a feeling you might struggle to keep that alive, could be wrong, but as it warms up the rot can get worse.
I dont think its actually a disease as such, more a physiological disorder.

 

Thank you all, and for the suggestions to think about.

I killed the mealy bugs, and there are no vine weevil larvae in the compost.
I've had it about 5 years and this condition only started a few months ago.

It's called Echinopsis oxygona, and apparently can withstand temps down to -7 C, but it hasn't been in those temps. It's been in a shady Perspex shelter until I noticed this starting and then I brought it into the conservatory.

I'll repot it I think, and see what happens.

It's got beautiful flowers in the summer ...

 

OH Ok Echinopsis, never had one do that over winter, they tend to like it a bit cooler in summer but I'd never let mine go below 0C in winter whatever they might say about its hardiness.

Hope it recovers.

 

I never let them get that cold either @pete, and it's been such a mild winter too, so it hasn't suffered cold temperatures.

Thanks for your help

 

Then I reckon its a combination of low temps and damp.
Above zero, but damp atmosphere.

 

I agree with Pete in that it's cool temperature and damp atmosphere.
I used to live in Essex and over there my cacti and succulents were a lot happier and easier to grow. Since moving to Devon I have to be a lot more careful with the watering especially in early spring and in the autumn and every year or so I will lose one or two. I have a Ferocactus and a couple of other cacti with similar marks on them. It doesn't seem to spread, it just is rather disfiguring. Perhaps keeping slightly warmer to give a drier atmosphere and reduce the possibility of condensation might help.

 

Thank you @pete and @NigelJ Your help is much appreciated.

 

Something I've noticed over the years during winter.
We get varying temps and sometimes the change in temp can be fairly fast.

As cacti contain a lot of water the body of the plant remains quite cold for quite a while once temps rise, this means that condensation forms on the plant, and the plant can be pretty wet for quite some time when a cold spell is followed by milder weather.

The bigger the plant the longer it stays cold, so the longer the condensation remains.