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Water Relations in American Elm Infected with Ceratocystis ulmi. W. E. MacHardy, Research Associate, Department of Plant Pathology-Entomology, University of Rhode Island, Kingston 02881, Present address of senior author: Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada; C. H. Beckman, Professor, Department of Plant Pathology-Entomology, University of Rhode Island, Kingston 02881. Phytopathology 63:98-103. Accepted for publication 22 July 1972. DOI: 10.1094/Phyto-63-98.

Changes in foliar water loss and water transport pathways as related to the appearance of foliar wilt and chlorosis after inoculation of American elm trees with Ceratocystis ulmi conidia were studied under controlled environmental conditions. The wilt syndrome is associated with reduction of water available to the leaves. The appearance of foliar symptoms accompanied or followed a decrease in water loss. At no time did infected trees show an increase in water loss. In check trees, the transpiration rate either remained unchanged or increased gradually. Distribution of dye in infected branches revealed that the decrease in transpiration was associated with blockage of infected water-conducting tissue. Dye distribution within xylem elements was uniform in noninfected twigs, but diminished progressively toward the shoot tips in infected branches. One-year-old twigs and greenshoots were the most common areas for vascular dysfunctions associated with wilt symptoms. Leaves rarely wilted when vascular elements of current shoots were healthy, even if water-conducting elements of older twig sections were extensively blocked. The extent, degree, and type (chlorosis or wilting) of initial foliar symptoms expressed are believed to depend upon the speed and extent of fungal distribution and development and subsequent vascular dysfunction within the terminals of infected branches.