Powdery mildew fungus could possibly cause the leaves of your apple tree to curl or be distorted, but you also would see a powdery film or residue on the leaves if this is the case.

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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; 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.