Distorted or puckered leaves are generally a sign of aphids attacking the plant, trouble is they are masters of camouflage, with their green bodies blending with the green of the leaves. They are normally found on the underside of the leaf or at the junction of the leaf and the stem.
Quite often aphids are accompanied by ants who use them in some way to produce food for their young, so if you cannot see the aphids themselves but there is plenty of ant activity on the plant that has affected leaves then you most certainly have an aphid problem. Give the plant a blast of water from the hosepipe to knock the aphids off.
Have you ever had this situation one day you have a tray full of strong healthy seedlings and everything looks rosie, then the next morning they are all lying on their sides? And you wonder what the heck is going on, the most likely cause is cutworms, who hide just below the soil surface during the day and then at night come out and chew through the seedling at the base. Either surround the seedlings with a cardboard collar or place some thin wire in a vertical position near each seedling, this prevents the cutworms from curling around the stems of the seedlings.
Your new young tree that had been doing so well suddenly loses all it leaves, at the wrong time of year and this is the first time you notice that there is a problem. The cause is most likely to be a boring insect grub and they can be an insidious problem as the first the gardener notices is all the leaves falling from the tree. One grub can completely eat its way around the trunk of the young tree in one season causing death to the tree.
Prevention is the best solution, when you plant the sapling place a collar around the base until the tree is well established. Also by looking out for leaking sap at the base of the trunk, which will indicate the entry hole for the larvae of the beetle, wasp, moth or fly that has attacked the tree. Another giveaway is a small amount of sawdust at the base of the tree. At this point, place pheromone traps near the tree in question, and insert a thin wire into the entry hole to impale the grub.