Mammillaria carnea, 2.5 years from seed. One day I noticed these black marks on it. I isolated the plant and during a couple of months these marks self-cured. The plant is in perfect shape and health now (after 1 year). What is this and how should I have treated it?
It looks like a fungal infection. Im glad yours was cured. When it happened to me, the cactus was gone in a couple of weeks.
It happened at the location of a sunburn when the sun was too intense for the season and then it spread randomly. If a fungal infection is superfecial, spraying with mouthwash actually helps a lot.