I'm guessing sunscald, if the tree was previously shaded. Otherwise it could be the work of insect pests, or borers. Also, when you trim apple trees, you must get the trim close to the trunk, leaving a trunk may invite sappy bark fungus, which can infect the tree and cause some of the bark to fall off. Those are just first things pop into my mind. I would have a arborist conversant with orchard trees have a look at this, a picture should suffice. I'm in the States, and I would approach a local university or my county agricultural agent.