Dark brown and rust colored spots on Obregonia

[Cacidy]

I got an Obregonia a few months ago and it had been doing well over the summer. I recently bottom watered with imidacloprid and I noticed some small rust colored spots on the tubercles. The tips of 2 or 3 tubercles have turned dark brown or black. I repotted the plant into some dryer soil after I saw this. Any ideas as to what it is? Should I cut the black tips off? I hope it is not rotting, the black part still feels pretty firm.

Thanks for the help!

[ElieEstephane]

I recently had similar occurences on a stenocereus marginatus. It started out small and spreaded to all the plant. I didn't wanna cut so i don't ruin the shape of the plant. So i would recommend getting a thin marker and marking where the spots are. If you see them spread cut awway immediately with a sterile knife and sterlize again after each cut. It will ruin the shape but save your plant.
Ps: im very jealous of your Obregonia.