Could be chlorosis, or lack of iron being taken up by the tree from the soil - often caused by poor drainage and/or dense clay soil.

@Chadnutz

I'm sorry I didn't see this until now.  Scrolling through topics, and no idea how I missed this one from mid-July.

I agree that it could be chlorosis, which is iron deficiency, but it's important to understand the reason for the deficiency.

The chelated iron you are feeding is a good thing, but the alkaline soil is the real cause.  If the soil is too alkaline, the iron is not available to the tree.  You have to have a soil ph range that allows the iron to be taken up by the plant.  If the soil it too alkaline, the plant just can't get the iron.

It's the same reason that Miracle Grow sells a plant food called "Miracid."

That basically lowers the ph so plants like azalea and rhododendron (and other species) can access the iron in the soil.

SO....you could attempt to lower the ph in the area of the yard where your tree is located.  That might help it a lot.  And it PROBABLY won't hurt your other plants either.

That is not a blanket statement.  If you have other plants you care about, you should look at what they are and what level of alkalinity/acidity they tolerate.

However, most ornamental plants  prefer neutral to slightly acid soils. NOT ALL.  But most.