There are two possible causes. I see that the apple tree is in a pot so hot weather and lack of water could cause the leaf curl and blossom drop. This is most likely if you live in a hot climate. The more common reason is apple leaf curl midge. This tiny midge insect has a brown to black body with clear wings. It climbs up from the soil where it overwinters and lays eggs in the folds of young apple leaves. The larvae that hatch feed on the juice of the leaves causing them to curl. This is often followed by brown leaf edges. There is no specific insect spray for the midge but if you pull off every curled leaf as soon as you notice it and do not compost the infected leaves you may be able to get ahead of the insect. Next year in early spring put a covering of landscape fabric under the tree to stop the midges from climbing out of the soil and onto the trunk. A general purpose fruit tree insecticide may also be used to help control the apple curl midge but results have been mixed. Don't worry your tree will most likely survive either causes of the leaf curl. Once you get that tree into the ground and out of the pot it may be less stressed and less likely to suffer from lack of water or insects.