I don't know the cause, but I have two possible explanations. The first is that the sudden over-abundance of water after a prolonged period of drought has caused the tree to split, as often happens with tomatoes when the skin cannot expand as quickly as the inside.
The second possible explanation is that the damage may have been caused by insect pests burrowing into the bark. This theory is based on the fact that there are two distinct areas affected, and not the whole trunk. Furthermore, looking closely at the photos I took, I notice that in the cracks there are lots of what look like insect eggs.
Of course it may just be that some insects have taken advantage of the crack to lay their eggs in what they consider to be a suitable place. Has anyone experienced this sort of problem before? If so, can you please advise me what I should do?
Elsewhere on the tree there is evidence of a different problem - aphids. Here you can see a "raiding party" of ants attacking the aphids. I think they "milk" the aphids of the honeydew-like substance they secrete.
The fruits of the tree are getting bigger now, but I wouldn't say that they look particularly good. Last year most of the apples were afflicted with the Bitter Pit, which is associated with lack of water and lack of nutrients. It may just be that my apple tree is located in an unsuitable spot.
Fortunately the pears are looking much better. This year my "Conference" tree has 11 fruits on it - not a huge number, I know, but more than twice as many as last year!