The leaves on my five-year-old...

The yellow leaves might mean that your tree has an iron deficiency called chlorosis, or it may be suffering from a fungus such as shot hole. Both indicate that your soil may have poor drainage, which could weaken the tree and make it susceptible to disease. Check the soil. If it’s a heavy clay, that’s probably your problem. If you think your soil is OK, make sure your tree is getting enough moisture. Water it deeply when the surface is dry but the soil three to four inches down is still moist. You also might want to contact your local county extension office to find out exactly what malady your tree has. It’s hard to diagnose the problem without seeing it.