Answer: The white patches may be a fungus, brought about by less than perfect growing conditions, or they may be evidence that your plant has an infestation of mealy bugs. Try looking at the patches with a magnifying glass to see if you can tell whether the patch is a collection of little insects with cottony coatings, or just a single blotch of hairy fungus. Mealy bugs can be very difficult to control, and the first course of action is to try to remove them with a cotton swab dipped in plain water or rubbing alcohol. Or, try prying them off with a blunt knife. (You won't do any more damage to your plant than the insects are already doing.) This may leave scars that will callous over, but it will help you save your plant. As a last resort, you may have to use an insecticide to rid your cactus of the pests. Mealy bugs travel very slowly, but they can leave eggs on all kinds of surfaces.
It may be time to thin your cacti to allow for better air circulation and fewer chances for fungal diseases to take hold.
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