Natural conditions sometimes cause an ash trees leaves to wilt, turn yellow, curl and defoliate prematurely, However, these symptoms are more often a sign that the tree is infected with a disease or infested by insect pests. Identifying parallel symptoms occurring alongside leaf drop helps render a definitive diagnosis of the problem affecting your tree. Knowing the problem helps you manage it.
Ash Yellows
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A major disease problem facing ash tree growers is ash yellows, a disease cause by a microorganism called phytoplasma. The earliest symptoms are diminished new growth. A healthy ash tree adds 6 to 8 inches of new growth, while infected ashes may only add an inch or less. What new growth does appear grows sparsely in tufts on the end of branches. Before the leaves defoliate, they turn pale green or yellow or prematurely take on fall colors. Ash yellow can kill an ash tree if the disease is not controlled.
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Verticillium wilt
Verticillium wilt is caused by a fungus living in soil that infects tree roots and then spreads to the rest of the ash. Early Verticillium wilt symptoms appear in July and August. Leaves turn yellow and have a scorched or burn appearance around the edges. Leaf defoliation is more common on green ash than on other ash varieties. If leaves are falling off your green ash, and you notice the other symptoms discussed, your tree is probably infected with verticillium wilt.
Ash Anthracnose
Ash anthracnose is another common ash-tree disease. A fungus causes anthracnose. Infection is most common after cool, rainy weather, as these conditions favor fungal growth. The earliest symptoms include purple or brown spots with a cream-colored center on new leaves. As the disease progresses, the spots create a brown mass on leaves. The leaves eventually defoliate and stems may die.
Other Possibilities
Diagnosis of tree diseases is difficult because many symptoms mirror those of other ailments. Extreme drought stress causes yellowing and eventual defoliation of ash leaves. If you do not notice any of the parallel symptoms associated with one or another of the diseases named here, adopt a more aggressive watering schedule and see if the ash returns to normal health. If not, it is possible that leaf defoliation is the result of less common ash tree diseases.