Hope these photos are helpful. They don't really show the yellow up to well. Thanks.
Thanks for your answer We have been watering a lot extra lately due to the extreme heat. Could the yellow possibly be a result of sunscald as that side of the tree is exposed to the hot afternoon sun and the yellowing is not on the other side of the tree.
It appears you went offline without seeing my most recent post. I'm going offline for the night soon, and I don't want you to have to wait too long. If you noticed the discoloration early in the spring it may be sunscald. Despite its name, sunscald isn't really the result of hot sun. It occurs when winter temperatures alternate between warm/sunny and freezing. Warm days cause the tree cells in the trunk to become active. Then if it freezes at night, those active cells are damaged or killed. The result is sunscald. It often looks dark and sunken on the tree. Young trees are more prone to sunscald than are older ones. young thin bark is more easily damaged by the conditions that result in sunscald. If you didn't have such conditions, then sunscald is unlikely. Also, sunscald doesn't damage leaves, so if the leaves have yellow in them, something else is responsible.
Trees can also suffer from sunburn caused by intense summer sunlight. This, too, happens most frequently in young trees. It would be unusual, but not impossible, for it to happen to a 15-year-old tree. Symptoms include leaves that turn reddish brown, then take on a burned appearance, and bark that becomes dark and sunken. The bark often breaks apart as the season goes on.
I hope this helps to clarify.Anna