What is blackening the base of my apples? And, what should I do about it? Many thanks.
Where are you located?
I don’t know for sure but I wonder if it is black rot. It is more of an issue in a warmer zone. This is just a guess.
https://extension.psu.edu/apple-disease-black-rot-and-frogeye-leaf-spot
Other terms to research are brown rot and blossom end rot.