Mealy Bugs and Scale
We got a call about a sick cactus in San Francisco, and I asked for photos, so this is what we got. The plant has gotten afternoon sun (as much as there is in SF) and watered once per month.
Here ya go. Let me know if you need more pictures.
You have an infestation of insects – both Scale and Mealy Bugs. They are damaging the crown of your plant. You need to treat this ASAP. We recommend Neem Oil (100%, not “rose defense” which is cut with petroleum products) which will kill the pests and help prevent fungal infections, and is safe to spray directly on cactus (although preferably not in full sun.) Spray thoroughly, and again weekly.
As the crown is damaged, you will need to watch carefully to make sure the plant doesn’t get soft, which could indicate infection. If it gets to that point, I would recommend bringing it in to us to take a look, but don’t wait so long that the plant has died.
Good luck, and give me a call, or stop by the nursery, if you have any other questions.