There are several things it could be so....
First, and I hope that this is the problem, it looks as if it may be a layer of fungus growing on damage that has been caused by some insect etc.
Try scraping the surface with a knife. if it comes away and leaves a clean surface beneath then thats all it is.
The tree may well grow on and recovor on its own.
However if it doesn't then that is one big area to have a true desease infection.
It could be as Potatoes says a dose of Fire Blight although the leaves look ok and they usually show the first signs of this.
Does the damage go right through the bark to the cambium layer. If not then the tree is fighting back on its own.
Check for Collar Rot fungus by cutting a piece of the damage off and smelling it. A distinct smell of vinigar gives this one away. That amount of Collar Rot damage would be a death nell I'm afraid.
If it should by this re-plant well away from the site if possable as it a soil born pathogen.
The last one is Canker itself.
Again that's a big area to have such a problem and to save the tree.
Both the last two can respond to copper based sprays but on such a small trunk under such an attack its fingers crossed I'm afraid.
An apple a day.....keeps me in work.