I'd guess cultural practices (soil/watering). O. humifusa, when it's not covered in drifts of snow, is used to varying levels of light (given its wide distribution in the US and Canada) - full sun shouldn't be an issue.

It's outside and I'm not doing anything for it. And soil isn't the problem cause I've had these plants for years and they've never done this before!

This post was edited by CactusBoss on Sat, Jul 20, 13 at 18:18

Looks like an old pad that caught the inevitable fungal rot they all seem to get. If the new ones growing off of this one are near the soil, this one will wither completely and next your you'll be able to take it out cleanly.

Jeff, these grow up there too I suppose?

I don't like this one bit. I thought fungal attacks are fatal to cacti? should I cut some pads to be safe?

This post was edited by CactusBoss on Sat, Jul 20, 13 at 19:38