Hi Lance,

I am hopeful your largest sycamore tree will make a comeback. It is already looking pretty good. Many sycamores in my county (Franklin) were similarly affected. Leaf dieback is a common occurrence for these trees with a late frost. 

You can use the fertilizer spikes in late fall if you would like to. Generally though, mature trees do not require fertilization. https://extension.umd.edu/resource/fertilizing-trees-and-shrubs-and-nutrient-deficiency-symptoms

The only other thing it could be is anthracnose, which is a fungal infection. See here https://extension.psu.edu/mysteries-of-our-native-sycamore and here https://bygl.osu.edu/index.php/node/2156. The leaves of your close-up image, magnified to 200%, appear fully formed and show no darker spots, so I am reasonably sure that is not the case.  If you have any concerns that the tree is infected, please send a leaf and branch sample to the OSU Plant Diagnostic Lab. You can also send images. https://ppdc.osu.edu/submit-sample/landscape/woody-ornamental-plants

I hope this information was helpful. Sabine

Thank you Sabine for the response.  I am encouraged by your comments.