"richard" wrote in message
...
My biggest columnar cactus is rotting from the top due I think to
bringing
it indoors last winter to the wrong conditions. It was about 18
inches
tall. I've cut of the top, applied white spirit to clean and
rooting
hormone for the anti-fungicide it contains but to my dismay it has
continued
to rot.
............. Till now it has been a very fast grower.

And therein could lie the reason.
Fast growing would seem to imply that the sap is/was flowing freely,
i.e. the plant is/was lush.
Cacti are not meant to be lush fast growers.
As soon as a bit of disease or rot occurs the lush plant will implode.
A hard grown plant will suffer a blemish and recover.

I've heard of applying the rooting powder to help seal a cut, but
never heard of applying white spirit.

The usual procedure for dealing with cut cactus sections (grafts) is
to place them in a dry area and let them self heal - and certainly
with-hold watering as that just encourages more sap to flow - and an
easy route for rot to spread.

--
ned