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Etiology

Role of the Mycoplasmal Disease, Ash Yellows, in Decline of White Ash in New York State. J. A. Matteoni, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address of Senior author: Research scientist, Agriculture Canada, Research Station, Vineland Station, Ontario L0R 2E0; W. A. Sinclair, department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 75:355-360. Accepted for publication 12 November 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-355.

A disease caused in Fraxinus spp. by mycoplasmalike organisms (MLO) and called ash yellows is described from observations of naturally infected and graft-inoculated white ash (F. americana). MLO in phloem sieve tubes were detected by electron microscopy or Dienes' stain. Witches'-brooms and deliquescent branching were diagnostic symptoms of MLO infection. Other symptoms included reduced apical and radial growth, early onset of apical growth in spring, elevated diffusive resistance of leaves, formation of abortive sprouts, dieback, and death. Trees with yellows were abnormally susceptible to cambial injury by freezing, which caused bark cracks and cankers at trunk bases. Witches'-brooms and deliquescent branching were present throughout the region where ash decline is conspicuous. In observation plots, 57% of white ash were found to be infected by MLO. Death rates of infected and apparently noninfected trees during 17 mo were 10 and 1.9%, respectively. The year of onset of decline, indicated by reduced annual growth, varied among trees on the same site, and the frequency distribution of trees according to year of onset varied among sites. F. pennsylvanica was also found infected and declining.