Have you noticed any ash trees in your neighborhood dropping green leaves? Several days of heavy rain over the last few weeks have allowed the development of a common ash disease called ash anthracnose. The disease is caused by a fungus, Gnomoniella fraxini. The fungus spends the winter on old petioles (leaf stalks) and branches. Spring rain splashes spores onto new leaves and the fungus quickly causes brown dead splotches, or lesions, on the leaves. In reaction, the tree casts off the infected leaves. Although emerald ash borer is spreading rapidly throughout eastern North America, early leaf loss is not a symptom of that insect.
Anthracnose makes some trees unsightly, but this is not usually a serious disease. New leaves will start popping out quickly to replace the lost ones. As long as we get a reasonably dry spell over the next few weeks, the disease is likely to disappear. In really wet years, some branches may be killed.
The slide show below shows the disease in a single ash tree. It is interesting that the ash trees in the background of the first slide do not have the disease. There is a lot of genetic variation in resistance of trees to anthracnose.
Thanks to Kimber Sternberg for bringing this to my attention.