With all the concern about Emerald Ash Borer (EAB), many people may assume that any ash tree with problems is being attacked by EAB. However, EAB has only been confirmed in Douglas, Jefferson, Johnson, Leavenworth and Wyandotte counties. One of the other problems we see with ash is Mycosphaerella Leafspot. Though this disease looks serious, it is not.
Mycosphaerella leaf spot causes small, brown spots that enlarge to become blotches and may result in early leaf drop. Defoliation this late in the growing season will not hurt the health of the tree. Therefore, because this disease appears sporadically and tree health is not harmed, we do not recommend treatment. Furthermore, treatment would have to be preventative and applied before the disease had infected the leaves. Applying a fungicide now would have no effect. (Ward Upham)