Can you post a photograph as there's lots of reasons - birds packing them, wasps attacking the ripe fruit. Scars can be caused by them being touched, rubbed by neighbouring branches......
Sorry not sure how to add photo's Could be wasps but fruit not ripe yet and nothing rubbing against. There are a lot of Ants & rather a lot of Earwigs on trees or around base
There are so many pests and diseases possible on apples and plums that we really do need a photo uploading - maybe someone you know could help you do that? Spraying pesticides on trees from which you will eat fruit is obviously best avoided. If you rake-up and burn or dispose of all the leaves after they have fallen, you will reduce the number of overwintering pests and diseases, many of which overwinter in the dead leaves. Using a 'winter tree wash' in December will kill many of those harbouring in cracks in the bark etc. If you also put down a mulch using well-rotted manure (at least a couple of inches deep) after raking the leaves each year, the tree will get a good feed which will help it fight off some diseases and make it less likely to get others.
A trowel in the hand is worth a thousand lost under a bush.
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fidgetbonesDerbyshire but with a Nottinghamshire postcode. Posts: 16,475
With plums or apples, Pheromone traps put up at flowering time reduces the number of grubs in the fruit. Most garden centres will keep these.
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Could be wasps but fruit not ripe yet and nothing rubbing against. There are a lot of Ants & rather a lot of Earwigs on trees or around base