So I just looked at my plants after a while being away and discovered that my pedro is turning brown on this one rib. I'm not sure if this is anything to worry about, I suspect it's just from lack of sunlight but I wanted to make sure. It doesn't look so bad on the other side and there is no blackness or bruising. I haven't watered my plants for a long time so I don't think they would be in danger of rotting.
Can someone please enlighten me?

I'm looking to have a few cuttings next season and then grow those into glorious new pedros! My bridgesii are coming along nicely although they don't have a huge root system yet. I'm really hoping that my pedro might flower some time but I know that is a slim chance.
. My other one did, however, so I'm hoping that is some testament to its strength and resilience. Also I had a peyote that somehow ended up rotting last year even though I hadn't fed it for a while, I guess it retained some moisture and he just died over time even though he looked so healthy. I was going to graft it onto my pedro but that experiment will have to wait.