I have a 3 year old Candy apple tree with 4 main branches but all the foliage is on the top of the branches with nothing at all on the bottom two thirds,how do I correct this,and when?
Apart from this it appears to be very healthy.
It's called "blind wood" and is typically a symptom of inadequate chilling hours, although with your UK location I find that hard to believe. In the tropics they get around this by pulling the branches of the apple tree down almost horizontal like this fellow in Uganda.
I have a few obscure apple varieties very prone to producing blind wood. I have found the cure is to prune it way back and also to summer prune to induce more branching. Belle Fleur Rouge in particular is extremely fond of blind wood. This has nothing to do with chill hours since my chill is 2000+.
My Cortland tree has more blind wood than other varieties and I think this has something to do with Cortland being more of a tip-bearer than a standard spur-bearer. I am not familiar with the "Candy" apple variety but there are genetic behavioral factors that could be at play, such as whether the variety is more of a tip-bearer versus spur-bearer. There are extremes on both ends and then there are all different combinations of behaviors in between the opposites, depending on the variety, as well as how the tree is pruned, environmental factors, etc.
Good point, I have noticed it to be more common in tip-bearing varieties. My Belle Fleur Rouge is an extreme tip bearing tree.