such symptoms can be caused by yeast or bacteria after minor mechanical or insect damage. The most expected pathogen - Pectobacterium cactucida, followed by a number of other pathogenic bacteria. See https://www.researchgate.net/publication/15062775_Bacterial_succession_in_necrotic_tissue_of_agria_cactus_Stenocereus_gummosus
You can apply antibiotics (tetracycline, etc.), copper, silver, or zink- based fungicides to stop the disease spreading.
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Article Bacterial succession in necrotic tissue of agria cactus (Ste...