Bitter pit

Bitter pit is thought to be caused by:
lack of calcium
and/or
too much nitrogen fertiliser
and/or
unsuitable variety for the soil
and/or
unsuitable rootstock for the soil (including dwarfs in poor soil or full-size rootstocks in good soil)
and/or
acidic soil types where calcium is unavailable due to the acidity mopping it all up
and/or
poor pollination (a poorly-fertilised fruit attracts less of the tree's resources - that's partly the cause of fruit/fruitlets being dropped prematurely)

Young trees often suffer badly from bitter pit* - if it's trying to grow new shoots and roots to establish itself they steal nutrients from the fruit.

Train and prune the tree normally and once the tree is mature the bitter pit should go away. With varieties prone to bitter pit it's often best not to let them waste energy on fruiting; get them to put their energy into forming a nice tree and then when pruning/feeding stops the tree will settle down after a couple of years.


*
In my experience, far more varieties suffer from bitter pit than 'the books' say. In fact, some varieties said to suffer from bitter pit suffer much less than some varieties that aren't listed as prone to it.

As one of the causes of bitter pit can be lack of calcium, I think I read somewhere that this can sometimes be due to not enough water available around the roots to enable the tree to absorb enough calcium from the soil.

So rather than adding calcium to the soil, ensuring the tree has enough water can go a long way to helping (provided this is the only issue causing bitter pit).

As your tree is new (with a young, comparatively small root system) on an allotment in a reasonably dry summer, this could easily be one of the main causes.

Yes, insufficient water availability can cause bitter pit. I kind of covered it when I said 'unsuitable rootstock for the soil'.
A stronger rootstock will find the water it needs, and by having to put a lot of energy into finding water it tends to stay fairly dwarfish. I have a very free-draining soil with poor nutrient and poor water availability but M25 and MM111 rootstock find what they need without any help from me, albeit the poor conditions mean they are semi-dwarf in size and do well at 7-10ft (2-3m) spacing which is the spacing usually provided for semi-dwarf.

MM106 and M26 don't have the strength to find enough water in my soil, with MM106 in particular being prone to bitter pit, small fruit and to dropping the whole crop onto the ground in summer dry weather due to water stress. My MM106 and M26 were fine in the earlier years when being fed and watered, but are lost causes without intensive care.

Mañy thanks for your replies. As I am stuck with the m26 rootstock and my soil is free drainiing . I think next year I shall feed and water more often, and see what the results are. I am still open to any more suggestions, again many thanks for all your help.

I agree with the others. I have a couple of vigorous Ashmead's Kernel and find the variety is one of the worst for bitter pit, along with Grenadier. However, usually less than 50% of the fruits are affected and the ones with bitter pit can often still be eaten and enjoyed. Sometimes the unsightly markings are not distributed throughout the entire apple, but restricted to the surface cm or so of the flesh.

This year has been very dry here, during the growing season. I would suggest extensive mulching round the base of your tree with grass clippings or compost next year to aid water retention.

On my soil trees on M26 really struggle to grow with non-vigorous varieties as scions, compared with those on MM106, so much so that they often 'runt out' after 5 or 6 years.