Condition of green ash, incidence of ash yellows phytoplasmas, and their association in the Great Plains and Rocky Mountain regions of North America.
Approx. 50% of 1057 green ash (Fraxinus pennsylvanica) systematically sampled in the Great Plains and Rocky Mountain regions of North America had substantial dieback (>10% of crown branches with dieback), and the average growth ring width during the last 20 years was 2.9 mm. The overall condition of the population was rated fair. Ash yellows phytoplasmas were identified at 102 of 106 sites throughout 6 US states (North Dakota, South Dakota, Wyoming, Nebraska, Colorado, Kansas) and 3 Canadian provinces (Alberta, Saskatchewan, Manitoba). These phytoplasmas had not previously been known in Alberta, Saskatchewan, Manitoba, Wyoming, Colorado, or Kansas. Incidence of phytoplasmal detection ranged from 16% in Wyoming to 71% in South Dakota. Incidence varied in the range 41 to 67% across site types and crown dieback classes. Incidence was highest in rural plantings, in trees with the most crown dieback, and in larger diameter trees. No significant relationships were detected between the presence of ash yellows phytoplasmas and radial growth rates of trees.