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Effect of Phytophthora Root Rot on Water Relations of Avocado: Interpretation with a Water Transport Model. R. E. Sterne, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside, CA 92502, Present address of senior author: Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701; M. R. Kaufmann(2), and G. A. Zentmyer(3). (2)Associate Professor, Department of Plant Sciences, University of California, Riverside, CA 92502, Present address: U.S. Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO 80521; (3)Professor, Department of Plant Pathology, University of California, Riverside, CA 92502. Phytopathology 68:595-602. Accepted for publication 22 September 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-595.

A field study was conducted to compare the water relations of healthy avocado trees and trees with roots naturally infected with Phytophthora cinnamomi. Data for diurnal fluctuations in stomatal behavior (measured as leaf conductance of water vapor), transpiration, and water stress for well-watered healthy and diseased trees are presented. A water flux model of the soil-plant-atmosphere continuum was used to examine data on leaf xylem pressure potential (determined by the pressure chamber method) and transpiration for healthy trees (well-watered and water-stressed) and for diseased trees. Values of leaf conductance and transpiration were higher throughout the day in nonstressed healthy trees than in those with Phytophthora root rot. Daytime and nighttime values of xylem pressure potential were lower in diseased trees than in healthy trees; the lowest potentials were 13 to 14 bars and 9 to 11 bars, respectively. Xylem pressure potential at night (zero transpiration) averaged 1.8 bars and 7.0 bars in nonstressed and water-stressed trees, respectively, and 8.0 bars in diseased trees. There was a consistent relationship between xylem pressure potential and transpiration in leaves of nonstressed avocado trees; i.e., changes in transpiration were accompanied closely by changes in xylem pressure potential. However, the relationship in diseased trees resembled the relationship in water-stressed trees: it was inconsistent and varied with the time of day. At a given transpiration rate in the afternoon, xylem pressure potential in diseased trees was considerably lower than with equivalent transpiration in the morning. The water flux model predicted that in diseased trees, low xylem pressure potential resulted from increased resistance to flow in the soil-plant system even in well-watered soil. A hypothesis for the increased resistance is given. Apparently, root disease caused by P. cinnamomi induced such major changes in the water transport system of avocado trees that typical plant control mechanisms (e.g., stomatal closure) could not correct the water deficits.