Why are the leaves on my tree turning yellow?

This could be an issue of "transplant shock" where a tree is losing some leaves due to the stress of being purchased, transported, and installed. Take note to see if the leaves that are yellowing and dropping are older, interior leaves. It could also be symptoms of a disease: Look for any lesions or dots on the leaves, which could be indicative of a fungal disease. Peaches (and stone fruits in general) are very susceptible to fungal diseases. They often require treatments during the growing season to ensure good health and fruit set. A balanced fertilizer that is slow release and contains small quantities of major nutrients and micro nutrients would be beneficial if applied in the fall.