Just recently ash trees around the area have been prematurely dropping their leaves shortly after the trees leafed out. Due to loss of leaves, affected trees have a thinner crown compared to nearby broadleaf trees of different species.
Two main events are causing this leaf drop: 1) frost/freeze events and 2) anthracnose fungal infection. Frost/freeze events occurring after the buds had started to swell or after buds had already broken damaged newly expanding leaves or caused tender leaves to die. Additionally, the fungal disease anthracnose, was able to infect some leaves this spring causing dead spots on the leaves. Ash trees will purge these damaged leaves.
When you look at ash leaves that prematurely fell to the ground, they will have black/brown blotches on green leaves and many of them are curled up or distorted. Photos above and at right show damaged leaves that the tree will choose to drop. The leaf disease, anthracnose, is commonly seen when the spring weather is cool and wet. Leaf damage tends to be more severe on the lower crown of a tree if anthracnose is the culprit whereas frost damage will be more severe in the upper portions of the tree and the outer edges.
Although ash may drop the majority of its leaves the tree will attempt to send out a second set of leaves and will look much better the rest of the season. Once leaves mature, temperatures increase, and spring shower season ends, the problem usually subsides. No control is necessary for yard trees although it is a good idea to maintain the overall health of the trees by watering during dry periods, properly mulching, and minimizing injuries to the tree.
UW Extension has a factsheet available on anthracnose at the following url: http://wihort.uwex.edu/gardenfacts/XHT1001c.pdf
For more information on the Village's urban forestry program, visit: http://www.villageofbellevue.org/departments/leisure-services/urban-forestry.html