Experts and BHG readers answer.
LEAVES CURLING ON APPLE TREE
"Powdery mildew fungus could possibly cause the leaves of your apple tree to curl or be distorted, but you also should see a powdery film or residue on the leaves if this is the case.
Various insects can cause leaves to curl on apples. Aphids and leafrollers are the 2 most likely candidates. Look closely inside the rolled or curled leaf for evidence of the insects and spray an appropriate insecticide if the condition is severe. If the amount of leaf rolling is minor, the tree should continue to produce a good crop without the need for a spray."