Is this cactus going to be alright?

[duckworth]

Hello everybody!

BACKGROUND:
This cactus was recently rescued (pictures below) from a small pot, and any help would be greatly appreciated! This champion is around 20 years old and has been yellow like this for a long, long time. It was in the broken clay pot you can see behind it, with bad drainage and soil that was ignored for 20 years.

My main concerns are these...

QUESTIONS:
- Will this cactus potentially die, especially since twas re-potted it twice (e.g. shock)?
- Will the main trunk heal and become green again?
- Will the pups handle the re-potting and new conditions okay, and grow well?
- If a cutting is taken and the cut is very close/flush with the trunk, and the calloused area hardens but shows black spots (mould?) - could this mould infect the rest of the cactus, since it is so close to the trunk?
----- A 4 inch cutting was taken which had potential, so that it can become its own plant (this is visible in the photos).

PHOTOS:
http://imgur.com/a/47RcR

DETAILS:
It was re-potted it about 3 weeks ago - Now it is in a 50:20:30 mix (Pumice+Perlite+pebbles : Fertilised Compost : Garden Soil) with large rocks at the bottom and many holes for drainage. The container is big, but the areas where there is no cactus get covered so that they stay dry. It was actually re-potted in this new container twice, because the first time didn't use enough pumice/perlite/rocks.

CONDITIONS:
- Re-potted in winter with temperatures of 5-12 Celsius (40-50 Fahrenheit).
- Winter temperatures generally as above; summer temperatures 15-22 Celsius (60-70 Fahrenheit).
- Very windy and dry all year round, except for rainfall.
- Has received a fair bit of rain over the years.
- Has been exposed to all day sun far from the equator - the sun here is harsher due to hole in the O-Zone layer. May have contributed to burning. We understand that this species prefers less sun.

Thank you very much for the help!

BONUS QUESTION:
With pups, to propagate you have 2 choices: you can cut them (a cutting), or you can twist them from the trunk. When you do this, the roots comes with it, and there is little to no raw/exposed plant flesh. However unlike a cutting, you don't get a large, flat base when it callouses. You just have the cactus' skin going inwards towards the root - it is rounded off.
Does this affect its ability to grow? Since the roots probably will only come from this limited space. - Unless I'm wrong on that one.

[greenknight]

- These are very tough plants, it will probably survive. I trust you broke up the old root ball and spread the roots out, you don't want circling roots.
- It may green up, or it may just put most of its energy into the pups; that main stem has a lot of damage.
- The pups are the part that's most likely to do well.
- Any wound could introduce rot, but the tissue of the joint is adapted to create a barrier to infection. If the cut has callused over, rot is unlikely. Black spots are sometimes harmless mildew, but keep an eye on it - if rot develops, surgery is the only treatment.
- I don't see any cutting in the pictures.

These can take full sun or partial shade. Not certain of the species, it looks like Echinopsis (Trichocereus) scopulicola, but E. pachanoi (San Pedro) is similar, and there are many hybrids. They're all tough as nails, anyway.

If pups break off easily, that's fine - it actually ruptures fewer cells than cutting, there's less chance of decay. Roots can erupt out of the epidermis, not just from the callus; they'll have no problem growing roots. If pups don't detach readily, though, you should cut. Cacti in this group grow easily from cuttings, you should have good success either way, but if you have to cut they need to heal longer before planting them.

I take it you're in the Southern Hemisphere?

[duckworth]

Thank you for the fantastic answer, greenknight! This has been very helpful. Yep the roots were spread, and yep, we are in the southern hemisphere. The sun is harsh here in New Zealand (when it shines). Maybe I'll lather this cactus up in sun block
Thanks again!