I have found neem oil directly applied to the infection is effective. Ethanol is too, but not as strongly.

Will wait and see whether feeding prevents a recurrence

Thanks, both. Repotting took place about 18 months ago. The serious yellowing is only to that single stem and to me looks to relate quite specifically to the scabbing. It is central and at the top, so it does get whatever sun is going. There are many stems both bigger and smaller and many of which f...

Thank you Steve. I am in Salisbury, southern part of UK. Probably equivalent to US zone 7 or 8. The plant is on a windowsill that gets direct sunlight for about 5 hours in the middle part of the day. It doesn't normally get very hot though and doesn't experience temperatures below 10 degrees. Last t...

Catus whole small.jpg Cactus part.jpg One piece of this venerable cactus seems to be suffering from the dark scaly disease at its tip and it is beginning to turn yellow. I am worried that the sickness will spread. The plant as a whole has some black spots with cork-coloured edges. It is not widespr...