I just received this Mexican Fence Post cactus for mother’s day. I just noticed yesterday that there is a black spot on the new growth. I only water when it’s completely bone dry. It got watered maybe 2 weeks ago. Last watering was the first time I used cactus fertilizer as was recommended to me on another forum. I’m new to caring for cactus so I’m not sure what to do at this point but I’m so bummed. I really don’t want to lose the Cactus so I appreciate any and all help. Currently it’s still in the plastic pot it came in, and that’s sitting inside a terra-cotta pot. It’s an indoor cactus as I live in a climate thats not conducive for cactus life the majority of the year. So it’s sitting in a south facing window.

oh, and I just checked my cereus that I got the same day that Inlast watered my MFP cactus. And it has black spots too. A lot more than this one.