Problems with my replacement Apple tree

4 years ago I planted an number of fruit trees. They seemed to do well, then one developed an area of brown dead looking bark which encircled the main stem. In May last year. The tree died. We put it down to the drought...

This last winter I bought a replacement tree, and planted it. Yesterday I noticed that the lowest side shoot had started to die, and there was a brown circle of bark spreading from the base of the shoot, beastly going all the way round the main stem.

Is it happening again? If so, what is 'it'?

As you can see, so far the tree still looks ok...

 

Arrgghh!!
I hope not! We did loose an apple tree to canker a couple of years ago. It could be that this variety is susceptible....