The cactus based turns yellowish brown. Should I worry?

[Lupask01]

My new astrophytum showed yellowish brown after repoting. Is this the early sign of rot?
I confirm that his roots are strong and healthy, at least to the amateur person like me.

[ohugal]

Does the base feel very mushy in comparison to the rest of the plant body? Try poking it with the back of a pencil or something. In case of cutting out the rot and rerooting it, there are a lot experienced growers here who might be able to help you. Good luck!