My ash tree is losing a lot of leaves this spring.
Several pests can cause the leaf drop you describe. Anthracnose, a fungal disease, causes brown spots on the leaves. The infected leaves eventually drop to the ground. Rake and destroy fallen leaves. This will reduce the source for future infections. Heavy infestations of plant bug can also cause leaf drop. Heavy rains and high winds are effective at keeping these insects under control. Leaf drop for no known reason also occurs on ash trees. It seems to be a problem for several years and then disappears. The trees seem to recover fine. Otherwise healthy ash trees will tolerate leaf drop caused by these three problems. Proper watering during dry spells, mulching, and proper care will reduce stress on the tree helping it cope with premature leaf drop. Consult a certified arborist (tree care professional) if the trees are also suffering from wilt, branch dieback and overall decline. These symptoms indicate more severe problems such as verticillium wilt or yellows.