First Report of Ash Yellows in Minnesota. D. W. French, Department of Plant Pathology, University of Minnesota, St. Paul 55108. S. Sundaram, B. E. Lockhart, and Edward Hayes. Department of Plant Pathology, University of Minnesota, St. Paul 55108, and Minnesota Department of Natural Resources. Plant Dis. 73:938. Accepted for publication 12 July 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0938B.
Ash yellows, a disease caused by mycoplasmalike organisms
(MLOs), was found in a woodlot in Olmsted County in southeastern
Minnesota. Most of the saw-log-sized black ash (Fraxinus nigra
Marsh.) in this stand have symptoms diagnostic for yellows: chlorotic
brooms often low on the main stem, slowed growth, dieback, and
mortality (1). Approximately 60 saw-log-sized ash are dying or dead.
In addition to the overstory mortality, many of the new generation
of ash are dying and dead. This woodlot is approximately 200 km
from the nearest known trees with ash yellows in northeastern Iowa.
Positive identification of the disease was based on field symptoms
and on electron microscopic detection of characteristic MLO structures
in phloem elements of diseased, but not of healthy, leaf petiole tissue.
Reduction in growth as determined by annual rings suggests the disease
has been in this stand for several years. Aerial photography and ground
checks have not disclosed ash yellows in any other stands in