It's a bud cutting from another plant last summer and is now 6.5" tall in the container. I don't over water it, but it's developed brown splotches on one side of a rib (at present doesn't show on the other side of the rib). Can anything be done?

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Looks big for the pot (ready for a new one now, or in spring if this is an indoor situation). I am guessing the bottom 4" was the cutting and the top 2.5" is new growth since it rooted. In any case that part looks different, newer, associated with lower light than the previous situation.

I do not know what the brown spots are. Hopefully somebody else here has insight about that. But I would just treat that as an imperfection rather than an active issue, maybe focus on good care and a bit more space.

We can get into the details there if you like. This is likely to be a large plant at maturity, given time.

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Unfortunately I can only say that those patches will spread, the plant will live but those brown rusty patches will most likely become permanent scars. I have quite a few of plants have those and I don't know what caused them, I suspect it is something to do with water, humidity, air flow and light exposure. I gave up in treating some of the plants and moved them outside full sun full rain, and surprisingly they become healthy, the new growth all looks clean and fine. My brother has one mammilaria that at the end grow new skin from inside and let the brown patches peelled off, that is the only plant that I have seen can fully recovered.
I will suggest you to let the plant has better airflow.
Below are some of my plant that have those, the new growth are all fine.




This gymno just get those in the past 2 months, and the rusty patches are growing/spreading.