I noticed this appearing on my cactus. I had cut the top off and planted it individually. Using a small stick, I lightly scraped off the brown spot. It peeled right off. The next day the place where I had touched the cactus and peeled off the brown/bubbled area turned black and was sunk in. I'm keeping fungicide on that area.
Any idea what this is? It's gone now but I seem to have damaged my cactus in the process.
