Apple Leaf Problem

An unidentified leaf issue has appeared on one of the apple trees. Just as with all other tree problems that involve yellowing leaves this one is mysterious and annoying. The game starts when you take a photo of your yellowing leaf and start comparing it to pictures of other yellowing leaves you find on the internet which are always slightly different. In the end you give up and just start adding more and more things to the soil in the vain hope it will go away. The only leaf yellowing condition I have turned up so far for apples is Apple Cedar Rust, a condition that can occur when apples are grow near cedar trees? Somehow I don't think so, there isn't a cedar arboretum nearby to my knowledge. Time to start adding things to the soil.