You don't mention the type of tree you have but I will guess it is either some type of maple or perhaps a pinoak. Both of these trees especially the oak are prone to a problem call chlorosis which you describe very nice. It is due to the fact the plant can take up enough iron to keep the leaves green and over time they get yellow, develp brown margins, spots etc and if really bad for many years branch die back. Often it is the result of plants growing in high pH soils, soils that are hot and dry for extended periods, soils that are compacted etc with the bottom line being the roots aren't workning as they should to get iron. While there are quick temporary fixes they don't address the real problem. if you can water deeply the entire area under the tree, apply a mulch under the entire area of the tree to keep the soil cool and moist, it can help to allieviate the stress and tree will grow better. Can use iron chelates to achieve a short term fix applied in the root zone or under the canopy of the tree.