My green ash trees have been dropping leaves this spring. Why?
The leaf drop is probably due to anthracnose. Anthracnose is a common fungal disease of trees in Iowa. Anthracnose may occur on ash, sycamore, maple, oak, walnut, and other deciduous trees (see encyclopedia article Anthracnose on shade trees). Cool, rainy weather in spring favors anthracnose development. Symptoms of anthracnose vary with the tree species. On ash, brown or black blotches typically appear on the leaflets. Affected leaflets often become distorted (they tend to curl toward the blighted areas) and fall from the trees.
Fortunately, anthracnose does not cause serious harm to healthy, well established trees. The affected trees will continue to leaf out. Leaves that develop later in spring are usually not affected as weather conditions are less favorable for anthracnose development. There is no need to apply a fungicide to affected trees.