Why are you growing it in pure pumice? Is it not yet rooted?

@Rob Blomquist No the yellow is definitely lack of nutrients. It has been 12 months in pumice with no fertilizer. The plant is very firm to the touch. But I agree the brown does look like rot and I will look at the roots. Assuming the root system is healthy, what would cause the rot to spontaneously appear near the top?

@cactusmcharris I find that cactus and euphorbia survive winters outdoors in the rain much better in a very fast-draining soil like pumice.

Not good Westes. It’s on a fast downward spiral. About a month ago i threw away the same exact Cereus as yours and it was pretty much the same size. It did the same thing. It eventually got all black after the yellow took over. I’d say cut off the tippy top that’s green, but I think you’d be wasting your time. The yellow is from the cold. It has rust which I only see during our damp winters.

I personally don’t think it’ll make it, but if it does keep us updated.

The yellowing is not due to lack of fertilization. These cacti require very little and it is entirely possible to grow them for years - even indoors - without any applied ferts and they will look unchanged......although may not grow as vigorously. If this was a nutrient disorder, the top would not still be green.

As noted, the yellowing is due to advanced rot or to cold damage. As this species is moderately cold tolerant and you state you have not experienced any severe or damaging cold, that would indicate rot as the most likely culprit.

I am also confused about your statement of moving from shade to sun. Why is this plant in the shade if grown outside in the Bay area? Your sun intensity, when you have it, would not require a shaded position and the plant would not be happy in much shade.

@gardengal48, I think the idea that the yellow is rot is wrong. This plant started out green and slowly became more yellow over time, always showing growth during that period. It has been yellow and without any sign of rot for at least 10 months. Once I put it into the sun at the start of last Summer all of the scale infestation disappeared. My theory would be that lack of nutrients weakened the plant, and this made it susceptible to the cold. In any case, we agree that the yellow does not give signs of a healthy plant.

To answer your question, I started this plant in the shade because it was a cutting, and I wanted to give it the best conditions for rooting. It was in the gritty mix at the time. It rooted fine, but in that shaded area the scale bugs found a delicious meal and badly infected both this cactus and its sibling. The sibling underwent a similar rotting, and I saved it after cutting off the top. At that point, with the plants rooted, I put them in pumice and put them into full sun. They thrived in that environment and the scale died back under full sun (something everyone here told me would not happen, but I got lucky).

I took the roots out today and they appear to be in excellent condition. I could not find any rot at the base of the plant. So - with fingers crossed - I guess I need to chop off the top of this poor plant and inspect inside to see how widely the exist rot has already spread. I guess the plant does not have a great prognosis, especially with all of this happening in the middle of Winter when the plant has low energy to fight infection. I hate to lose it.

How many inches below the low point of the rot should I cut, as my first try?

To answer your last first, into healthy tissue. Seal the cut, making sure the last cut is made with a sterile knife, with flowers of sulfur and keep dry.

To address the pure-pumice medium, there's no real need to grow plants in it if they're rooted. IMO, it deprives the plant of better growth with such a spare growing mix. Rooting them up in it is gangbusters, but once they're rooted, transferring them to a soil that has some top soil/loam in the mix is beneficial to the plant in the long run. Keep the pumice in it, of course, but for better growth (and fewer chances of this happening, as the plant's healthier and able to ward of attacks with greater effect).

@cactusmcharris, The problem I was having with Euphorbias in the winter rain is they would all get root rot in the gritty mix. Pumice is the only medium in which I can get really sensitive plants to survive winter rain. Now which cactus need that and which are tougher plants that can survive in gritty mix in the winter rain? The truth is I do not have the experience yet to know. I was just being cautious about it. This experience with the weakened plant spontaneously developing a rot at the top instead of the bottom is yet another hard lesson learned. I should never have allowed this plant to turn yellow heading into the winter.

Okay, so I did the cut, and the good news is there was not any rot in the mid stem of the plant. See the cut top surface of the bottom part of the cactus in the photo.

See separately the photo of the infected section that I cut off. Notice how the rot is completely at the surface of the plant and had not even penetrated to the mid stem yet. This is quite strange. Is this the case of an opportunistic infection at the surface taking advantage of a weak plant? I really do not see how rot got into that section.

Now that I have seen the nature of the infection, I am wondering would I have been better offer just cutting out the infection from the surface and leaving the core of the plant untouched? It would have created a big obvious injury, but really I am wondering if it would have survived that.

I did put sulfur powder onto the top of the plant after cutting it.