Michigan Ash Disease Mystery Solved

By Linda McElreath
January 28, 2002

A fungal disease caused the demise of ash trees in Michigan’s nursery stock last summer, Agricultural Research Service scientists have found.

Ash trees became popular for street plantings in the United States after an epidemic of Dutch elm disease killed thousands of elms in the early 1960s. But in Michigan in 2001, young ash trees of the Fraxinus americana (Autumn Purple) and F. pennsylvanica (Champ Tree, Cimmaron and Urbanite) varieties began showing signs of disease in the form of smooth, round, brownish-yellow cankers with distinct reddish, cracked margins.

Although ash trees suffer from many insect and disease problems, arborists in the region weren’t sure of the cause of the decline or why the problem was centered in Michigan.

Mycologist Amy Y. Rossman of ARS’ Systematic Botany and Mycology Laboratory in Beltsville, Md., isolated a fungus found on the diseased trees. She and colleague Lisa A. Castlebury used molecular sequencing to show that the fungus was Phlyctema vagabunda, known primarily for causing bull’s eye canker of apples. P. vagabunda had been reported on various hardwood and herbaceous hosts from temperate regions, but not on ash trees.

The fungus leaves a characteristic round canker of dead bark that may become discolored, sunken or cracked or may fall away altogether. Often the rest of the branch beyond the canker dies or becomes much less productive. Cankers can attack the bark of trees of all ages.

Identifying the P. vagabunda fungus as one already present in the United States reduced fears that it might have been brought in from outside the country and could spread even further, or that it would prevent other countries from accepting certain U.S. agricultural products.

ARS’ identification of the pathogen is the first step toward developing control measures to slow or prevent the decline of ash trees in Michigan.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.