EPIDEMIOLOGY OF A SLOW-DECLINE PHYTOPLASMAL DISEASE - ASH YELLOWS ON OLD-FIELD SITES IN NEW-YORK-STATE

Incidence and increase of ash yellows were studied in six white ash (F raxinus americana) populations on four sites in central New York State in 1990-1994. Each of 110-307 ash per population was observed for sym ptoms and tested with DAPI for detection of phytoplasmas annually for 3 or 4 yr. Yellows incidence based on symptoms was correlated with inc idence based on DAPI tests (r = 0.89). Symptom detection usually lagge d 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 p er year for the DAPI test was estimated to be 94% based on the frequen cy of positive results for 2 consecutive years in the same trees. Init ial incidence of ash yellows based on DAPI test results varied among p opulations from 5 to 45%. Rates of annual increase averaged over 3-4 y r 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 t he initial population per year based on DAPI tests and 4.0% based on s ymptoms. Higher incidence and higher rates of increase occurred in pur e ash populations than in ash mixed with other species. New infections and new symptoms were detected more often in trees with crowns expose d to the sky than in shaded trees. Disease incidence was not significa ntly greater in crowded than in scattered ash trees. A proximity test revealed no significant difference between the average distance from n ewly diseased to previously diseased trees and the average distance fr om newly diseased to previously healthy trees. Pathogen transport over meters to tens of meters from diseased to healthy ash was indicated b y proximity test results, similarity of disease incidence in clustered and scattered trees, and greater incidence of yellows in trees with c rowns exposed to sky than in shaded trees on the same sites.