Dorset,
United Kingdom 
the apples have grown but some have rotted on the tree
25 Aug, 2010
Answers
Agreed but would add, remove all the rotten ones and bin them. If it is brown rot then the spores will over winter on the tree bark and you will get the same problem in future.
25 Aug, 2010
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Cut one open, see if there's a caterpillar or obvious signs of one having been there inside. Could be Brown Rot, could be caterpillar infestation or a couple of other things.
25 Aug, 2010