Frequently I find that my cacti seem to suffer from looking ill. Instead of
being bright green they get a yellow look, and yet any offshoots grow
without this appearance. The main original stems of prickly pears often
become yellowed, and even flaccid as well, whereas their offshoot leaves
seem perfectly fit.

I also have an Aporocactus Conzatii where the main stems are yellowed, but
the side stems are not.

This feature only seems to happen to plants that are stem-like rather than
being globular.

I can't make out what's going on. Can anyone help?

Rob Graham