I have recently seen this appear on my cactus it is hard to the touch. Could you tell me what it is and how to treat if I can.
From the photo it looks like it could have been beetles chomping on your cactus, or a simple case of winter rot. Since you say it is now hard to the touch you probably don’t have to do anything, however I would spray with hydrogen peroxide and follow up in a week with an organic fungicide like Neem Oil (never use any neem product called Rose Defense) just to be sure. On the other hand if you see it spreading then send me another picture, a closeup in good focus.