Without a photo, its hard to be sure, but it could be a fungal infection known as pyracantha scab. Have a look here to see if the photo and description match what's happening to your plant

https://www.rhs.org.uk/advice/profile?PID=265

Could also be fire blight. Treat with a fungicide and cut out any dead. My understanding is that when people had coal fires the air became full of sulphur and that tended to cover many plants and acted as its own fungicide. Didn't do too much for our well being though.

Branches affected with fireblight normally don't recover, though new growth may fill in from below.