Why are the leaves falling off my ash trees?
Ash trees can suffer from a variety of problems. Without more information and good photos of the entire tree and close up on branches and leaves, we can only guess. Tree age, sun exposure, construction events in the tree vicinity, good descriptions of the leaves, branches, trunk all would be helpful.
Here is a link to the University of Minnesota's self diagnostic tool called "What's Wrong with my Plant"? http://www.extension.umn.edu/garden/diagnose/plant/ You may be able to self diagnose the problem.
A guess as to why your ash leaves are falling is ash anthracnose, a fungal disease that afflicts ash and other trees in cool, damp, spring weather. It results in leaf drop and the leaves will typically have brown or black blotches that appear on the leaflets that often become distorted, curling toward the blighted areas.
Fortunately, anthracnose does not cause serious harm to healthy, well established trees. The affected trees continue to leaf out. Leaves that develop later in spring usually are not affected, as weather conditions are less favorable for anthracnose development. There is no need to apply a fungicide to affected trees.
Read about ash anthracnose here: http://www.ag.ndsu.edu/news/newsreleases/2010/may-17-2010/anthracnose-hitting-ash-trees-hard
Emerald ash borer hasn't been found in your County yet.
Thank you for contacting Extension.