Probably aphids inside the curled-up leaves. You can just snip-off the worst affected ones and the tree will grow new ones. Use the spray on the hose to clear-off any others you see, which will nearly always be on the newest leaves at the tips. Hanging a bird feeder nearby may help as the birds will clear up aphids while they await their turn at the feeder.
A trowel in the hand is worth a thousand lost under a bush.
Why spray with an insecticide if you’ve already removed the most affected ones @BenTheBeginner?
You’re surely not suggesting using a systemic insecticide on a food crop 😱
leaves with just a few aphids are easily dealt with between thumb and forefinger
Hi, sorry I assumed this tree is too young to fruit, my apologies, if you're planning to eat the fruit please don't. In my experience aphids have only been managed via insecticide, but aphids have never been numerous on my Apple Tree, again apologies
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leaves with just a few aphids are easily dealt with between thumb and forefinger