Apple tree leaves started to turn brown/black around edges and dropping off
Thanks for the question. My first thought based on your description and the picture is over fertilization. Brown edges are usually a result of some "root" reaction. New trees, despite some recommendations, should not be fertilized too early. Fertilizer spikes can be the worst. If you did not apply fertilizer then it could be a "wet feet" issue. Again with the roots being saturated.
I hope this helps.