Ash Yellows in Velvet Ash in Zion National Park, Utah: High Incidence but Low Impact. W. A. SINCLAIR, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. H. M. GRIFFITHS, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; and M. TRESHOW, Biology Department, University of Utah, Salt Lake City 84112. Plant Dis 78:486-490. Accepted for publication 15 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0486.

Possible causes of slow growth and dieback of velvet ash (Fraxinus velutina) in Zion National Park, Utah, were investigated. Mycoplasmalike organisms (MLOs) were detected by means of the DAPI (4’,6-diamidino-2-phenylindole'2HCI) fluorescence test in 5, 7, and 45% of velvet ash sampled in three localities but were not detected in singleleaf ash (F. anomala). In Zion Canyon, MLOs were detected in 51% of velvet ash ≥6 cm dbh and in 35% of saplings. Branch dieback occurred in 74% of trees ≥6 cm dbh in which MLOs were detected and in 56% of noninfected trees. Low titer of MLOs in velvet ash was indicated by sparse fluorescent particles in DAPI-treated root phloem observed by fluorescence microscopy and by 39% incidence of negative DAPI test results in trees retested 1-2 yr after MLOs were first detected in them. Witches'-brooms and dwarfed, simple-leaved shoots near ground level, diagnostic for ash yellows in eastern ash species, were uncommon. Tolerance of velvet ash for MLO infection was indicated by scarcity of diagnostic symptoms, similar vigor of MLO-infected and noninfected saplings observed for 3 yr, and continued vigorous growth of MLO-infected velvet ash, but not white ash, seedlings after graft-inoculation with a New York strain of ash yellows MLO. Many velvet ash in Zion Canyon sustained severe foliar damage by unidentified loopers (Lepidoptera: Geometridae), ash plant bugs (Tropidosteptes pacificus), and lace bugs (Leptophya sp). Insect injury, rather than MLO infection, was a likely cause of declining vigor and dieback. Mature trees on some sites had also been damaged by water deficiency.