Structure and Composition of Forest Stands Affected and Unaffected by Ash Yellows. Peter J. Smallidge, Graduate Research Assistant, State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse 13210-2788. Donald J. Leopold, and John D. Castello. Associate Professor, and Associate Professor, State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse 13210-2788. Plant Dis. 75:13-18. Accepted for publication 3 May 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0013.

The incidence and effect of the mycoplasmal disease ash yellows in forest stands were evaluated in 50 plots located in six northeastern states. Three forest types identified by a vegetation analysis were dominated by Fraxinus americana, Acer saccharum, or F. pennsylvanica. The density of dead F. americana was greatest on sites dominated by this species, and the height of dead F. americana relative to live trees, or vertical distribution, differed among the forest types. Total tree mortality was generally greater in yellows-affected plots than in unaffected plots because of greater F. americana mortality. Live tree species composition was similar between yellows-affected and unaffected plots. The density and basal area of dead overstory F. americana were greater in yellows-affected than in unaffected plots dominated by F. americana. Mortality in plots dominated by F. pennsylvanica was minimal and restricted to individuals below the main tree canopy. Only one of 15 plots dominated by A. saccharum contained ash infected with mycoplasmalike organisms. A significant association was detected between the occurrence of ash yellows and both herbs characteristic of exposed conditions and areas classified as exposed. This association may explain the high levels of mortality in previous studies.