Cactus rot or disease??
Hi - My cactus recently dropped off a couple of lobes, and upon inspection, I realized that some of sections were covered in what looks like a layer of sand. This happened fairly quickly, maybe less than a week? I rarely water this cactus, but it has been extremely humid here lately. The cactus is kept indoors, in a bright window (south facing). In one of the pictures, you can see where a piece has broken off. Any idea what this could be, if the plant can be saved, or if I should be concerned for the other plants in its vicinity? Thanks!
Here is a link that might be useful: