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Stomatal Behavior and Water Relations in Sugar Beet Leaves Infected by Erysiphe polygoni. T. R. Gordon, Department of Plant Pathology, University of California, Davis 95616; J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 72:723-726. Accepted for publication 25 September 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-723.

Sugar beet plants (Beta vulgaris L.) infected by Erysiphe polygoni (cause of powdery mildew) on their adaxial leaf surfaces showed altered stomatal behavior. In the dark, both diffusive conductance to water vapor loss and viscous flow conductance of mildew-infected leaves were increasingly higher with time after inoculation, as compared to healthy leaves. This indicates that stomata on infected leaves failed to close completely in the dark. Water stress was induced in healthy and mildew-infected leaves by withholding water from the soil in which plants were growing. Illuminated healthy leaves showed a substantial decrease in leaf conductance to water vapor loss as leaf water potential dropped, whereas for mildewed leaves conductance to water vapor loss in the light was not significantly affected by leaf water potential values as low as 39 bars. As a result, mildewed leaves transpired more than healthy leaves at low leaf water potentials. At soil water potentials less than 6 bars the water potential values of leaves on heavily mildewed plants were much lower than leaves on uninfected plants. The relationship between turgor pressure and leaf water potential was not altered by the disease.