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Some Effects of Uromyces phaseoli on the Transpiration Rate and Stomatal Response of Bean Leaves. J. M. Duniway, Former Research Assistant, Department of Plant Pathology, University of Wisconsin, now National Science Foundation Postdoctoral Fellow, Research School of Biological Sciences, The Australian National University, P.O. Box 475, Canberra City, A.C.T. 2601, Australia; R. D. Durbin, Laboratory Leader and Professor, Pioneering Research Laboratory, Crops Research Division, ARS, USDA, and Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 61:114-119. Accepted for publication 10 August 1970. DOI: 10.1094/Phyto-61-114.

The transpiration rate and average stomatal aperture of primary bean leaves were significantly reduced during the fleck stage of rust development. A leaf disc assay for stomatal response showed that the pathogen inhibited stomatal opening in the light. Although inhibition was greatest during the fleck stage, the effect persisted through the remainder of the disease cycle. There was a linear decrease in the average stomatal aperture attained in the light as the infection density was increased up to 75 pustules/cm2; at higher infection densities, there was no additional change in stomatal aperture. When the effect was maximal, the average stomatal aperture was about 30% of the healthy control. A significant reduction in stomatal aperture occurred up to 0.5 mm from the margin of isolated fungal colonies. Unlike the control, the stomata of diseased leaf discs did not respond to CO2-free air. At sporulation, the epidermis was ruptured and cuticular transpiration became paramount. This was evidenced by the high transpiration rate of rusted leaves in the dark, and by the lack of a correlation between stomatal aperture and transpiration after sporulation.