Ferocactus disease. Help!!

Hi guys. I have a problem with a few of my ferocacti. The spots are very slow to spread but are definitely spreading. Any insight would be greatly appreciated. It is not mite damage and does not appear to be scale. I was thinking some kind of dry rot, perhaps. Especially the fero latispinus. This poor fella has a deep hole in the side of it. As it rots I dig it out. The rot is not a wet mushy rot though. Its as if the plant is slowly turning to cork.

If several plants have the problem and it's not bugs, could it be the environment
- too damp, insufficient air/light etc. ?

Plenty of light and bone dry apart from when I choose to water it. I am at a loss. Maybe it is too dry?

How are the fero's doing? Have you found anything to slow it down?

I gave them a big drink and they seem to be doing ok. The latispinus is still growing but I haven't had a dig around in the hole for a while to see if the disease has progressed. I am considering treating with a trace elements soak to see if that will boost their recovery.