It does look like sunscald. The high temperatures and gator bag are the most likely cause. It is very possible it is getting too hot under the plastic and cooking the tissue. I would remove the bag until fall and mulch under the tree and make sure it gets an inch of water per week. Check soil moisture before watering and don’t water until the top two inches of soil are dry. The weather we have had and will get makes watering adequately but not too much a challenge.