We don't see a ton of black rot in Michigan, but it is possible. Pruning out infected branches is the best course of action. Also, be sure to fertilize and water trees properly to minimize stress. We typically do not use fungicides against this disease. Here is a good article:

https://extension.umn.edu/plant-diseases/black-rot-apple#:~:text=Black%20rot%20is%20caused%20by,fruit)%20attached%20to%20the%20tree

You can take pictures and send them to me to make sure it is black rot.

Nikki Rothwell