Ask Mr. Smarty Plants

We found an article on Chlorosis in Trees and Shrubs from Washington State University from which we extracted this paragraph that sounds like the symptoms you are reporting:

"Plants with iron chlorosis first turn yellow-green to yellow between the veins, with the veins remaining a darker green. With more severe chorosis the leaves  become pale yellow and develop brown spots between the main veins. Leaf margins may also turn brown with the leaves later drying up and falling off. Tree growth slows to a stop and dieback of branches can occur when iron chlorosis is extremely severe."

You have found good information in your research; however, we feel that the biggest problem causing chlorosis, especially in trees, is our highly alkaline soil in Central Texas. Here is a previous Mr. Smarty Plants question and answer on that particular problem. The previous question is concerning a different woody plant, but the principles are the same. Our take on all this is that not only is our soil alkaline, but has a lot of clay, which severely limits proper drainage around the roots.

This USDA Plant Profile Map shows that Platanus occidentalis (American sycamore) is native to this area, so you would expect it to be somewhat tolerant of our soils. In the Growing Conditions on this tree (please read all of our webpage by following the above plant link) this comment appears on preferred soil: "Soil Description: Moist, sandy loams or silty clays."  While your ideas on nutrient supplements are good, the basic problems of alkalinity and clay soil, which drains very poorly, still remain. Adding amendments to the soil when the tree roots are unable to access them is not going to be worth the time and expense.

Here is one more article on Iron Chlorosis, which discusses some treatments that are pretty extreme, and probably can't be attempted by a home gardener. One thing we observed from our research is that the sycamore is a moist soil tree, most often occurring naturally in riverbeds and forest areas. With our extreme heat and drought this year, all trees are somewhat stressed. We recommend you begin with attempting to get more moisture to the roots, sprinkling out some distance from the trunk, as the roots are farther out in the soil than the dripline of the tree. Second, try mulching the roots with a good-quality shredded bark mulch. This will help keep moisture in, cool the roots and, as it decomposes, improve the texture of the soil in terms of better drainage.

It is very difficult to address poor drainage in a mature tree, so taking some short-term measures to help the tree survive is advisable. Then, hopefully gradual additions of compost and/or mulch to improve drainage will improve the life expectancy of the tree. If you wish to try injecting some iron and manganese in the soil this may help in the long run as the soil improves and allows the roots to access those elements.