Answer: The usual cause for black areas on the bottom of cacti (basal rot) is overwet conditions in the winter. Transplanting probably had nothing to do with the sudden appearance of the symptoms. Unfortunately, there's no way to save the diseased bottom of the cacti. If your cacti has an upper stem, detach and pot it up. Then sprinkle a layer of decorative pebbles over the soil surface to keep wet soil from touching the base of the plant. Water only during the spring, summer and autumn months, allowing the soil to remain dry during the winter.
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