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Ecology and Epidemiology

Epidemiology of a Slow-Decline Phytoplasmal Disease: Ash Yellows on Old-Field Sites in New York State. W. A. Sinclair, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; H. M. Griffiths, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 85:123-128. Accepted for publication 17 October 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/Phyto-85-123.

Incidence and increase of ash yellows were studied in six white ash (Fraxinus americana) populations on four sites in central New York State in 1990–1994. Each of 110–307 ash per population was observed for symptoms and tested with DAPI for detection of phytoplasmas annually for 3 or 4 yr. Yellows incidence based on symptoms was correlated with incidence based on DAPI tests (r = 0.89). Symptom detection usually lagged 1–2 yr behind phytoplasma detection, but 23% of ash infected for 3–4 yr did not show symptoms. Reliability of sampling one root per tree per year for the DAPI test was estimated to be 94% based on the frequency of positive results for 2 consecutive years in the same trees. Initial incidence of ash yellows based on DAPI test results varied among populations from 5 to 45%. Rates of annual increase averaged over 3–4 yr varied from 0 to 8% of the initial populations and were not related to initial incidence level. The average rate of increase was 4.9% of the initial population per year based on DAPI tests and 4.0% based on symptoms. Higher incidence and higher rates of increase occurred in pure ash populations than in ash mixed with other species. New infections and new symptoms were detected more often in trees with crowns exposed to the sky than in shaded trees. Disease incidence was not significantly greater in crowded than in scattered ash trees. A proximity test revealed no significant difference between the average distance from newly diseased to previously diseased trees and the average distance from newly diseased to previously healthy trees. Pathogen transport over meters to tens of meters from diseased to healthy ash was indicated by proximity test results, similarity of disease incidence in clustered and scattered trees, and greater incidence of yellows in trees with crowns exposed to sky than in shaded trees on the same sites.