Are you sure that's an apple tree and not a peach? That looks more like peach leaf curl. Apples can get some curled foliage, typically due to insects, but seldom will they ever develop that red puckering. That is characteristic of peach leaf curl fungus.

Try unrolling the leaf to see if there are any signs of insects curled up inside.

Might be aphids under leafs.

Alright thanks for the replies. It is an apple tree and yes apparently according to my father, there were insects inside the leaf. He said that it is not aphid, but more like a "very little light colored (slightly blue?)" fly. I am still looking at pests from internet, but I have not found the culprit yet.