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Hello everyone, I've bought this grusonii more than 5 years ago. It was growing pretty well, until last year some weird spots started appearing. My first thought - those were burns because it was staying on the direct sun and I moved it into the shadow, but then this year new spots appeared. They are located closer to the spines and I have no idea about the causes. Please help.
Images below.

This can be a slight sunburn. No one is perfect. Professionals shade their greenhouses to the needed level.

Sometimes sunburns are followed by secondary fungal infections that progress with time. Can not be removed. But this is much better than keeping such a plant in shade where it should start slowly dying. No need to panic about this.

Thanks a lot. Didn't know about that possibility of fungal to have delayed effect.
I've read that Echinocactus grusonii could be kept in partial shade while young not to have sunburns. For now I keep it over there almost half a year and it grows pretty well, maybe spines slightly smaller.