The decline seems similar to a bacterial disease called fire blight. Often, with fire blight I am looking to see the progression from branch tips which form a tell-tale "shepherd's crook" at the end. I am not seeing this here, but I do see blacked stems and the dead leaves are holding onto the tree, again additional symptoms of fire blight. While that is my guess at the cause of the dying leaves, the way to confirm this is to submit samples to the University of Illinois Plant Clinic. 

Treatment for fireblight includes pruning out infected branches and sterilizing your pruners between each cut. The symptoms do appear to be throughout the entire tree making pruning impossible or at least you will be left with not much of a tree after. 

There are no chemicals that can be sprayed to cure fire blight only those to prevent the spread. Those are often applied in the spring and dormant seasons. This article gives more information on fire blight and treatments. https://hyg.ipm.illinois.edu/article.php?id=1100

Again I would recommend submitting a sample to the Plant Clinic for a confirmed diagnosis.