Matucana growers - help needed

[JoZoo]

I'm repotting my Matucanas at the moment and noticed this unusual marking at the base. At first I thought that it was rot as it seems softer than the top part of the plant but when I looked at the roots seem look fine. Looking at other Matucana ritteri it looks like this could be a sign of ageing as they have the same shaped marks on the outer layer that has turned brown and hardened off. Is this normal for this plant? Is it some kind of corking?

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[Wilk]

Here's my M Ritteri. The plant is obviously healthy so I assume it is natural corking:

[RICHAUD]

all the plants of this group of Matucanas are corking :ritteri aurantiaca aureiflora .........

[Mike P]

I always wondered if this was caused by low temperatures and or high humidity during winter.