Shade tree decline is a complex disease situation since the cause frequently cannot be traced to one single factor. Typically, one or more primary stresses cause the health of a tree to deteriorate, and then secondary pathogens and/or insects contribute to further decline. Determining causes of decline requires careful examination of the tree and growing site as well as knowledge of the tree's history. Even then, diagnosis may be difficult because the original stress may be obscure or no longer present. Trees most commonly affected in Kentucky are Norway and sugar maples, ash and oaks (particularly pin oaks).
Hartman, John and Witt, Mary L., "Shade Tree Decline and Related Problems" (1991). Agriculture and Natural Resources Publications. Paper 34.