Question
My Mexican fencepost cactus is approximate 60 inches tall and the cactus is turning yellow from the base to one fourth of the way up.    What is happening to my beautiful plant?

Answer
Dear Reenie,

Is the yellow part of the cactus softer than the upper part?  Have you had much rain or moved it recently?  Usually yellowing that high up indicates rot that starts in the roots and moves up.  Bacteria or fungal infection enters a plant by some small break in the skin, usually by trauma and also by too much water and sometimes by root mealy bugs.

Check the condition of the yellowing and look at the roots.  If the base is indeed soft and/or the roots look black and stringy there's not much you can do to save that pat of the plant.  In that case, I'd take a sterilized sharp knife and cut above the rot to healthy tissue (if you need more than one cut, re-sterilize the  knife), let that callous and start a new plant.

Hope that helps any if you have any questions and /or a photo, please don't hesitate to send it to me.

Maureen