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Impact of Root Infection by Phytophthora cinnamomi on the Water Relations of Two Eucalyptus Species that Differ in Susceptibility. P. Dawson, Postgraduate research student, School of Botany, University of Melbourne, Australia; G. Weste, senior associate, School of Botany, University of Melbourne, Australia. Phytopathology 74:486-490. Accepted for publication 19 October 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-486.

Water relations changes associated with infection by Phytophthora cinnamomi were studied in inoculated plants of Eucalyptus sieberi (susceptible) and E. maculata (field resistant). Plants were inoculated and maintained in root boxes in a controlled environment. Growth of inoculated roots ceased and root lesions formed 8- 10 hr after inoculation. In E. maculata, neither the pathogen nor the initial lesion extended within the root and no other symptoms developed. In E. sieberi, the lesions extended and shoot wilting symptoms were observed 3- 15 days later. Along with lesion extension, a major reduction in the net hydraulic conductivity of root systems developed 2- 14 days after inoculation, although the fungus was present in only 8- 15% of the root system. Shoot wilting occurred when root conductance was reduced to ~9% that of the controls. Leaf conductance to water vapor loss, transpiration, leaf xylem water potential, and leaf relative water content were all reduced significantly 24 hr or more after major reductions in root conductance were detected. These changes in water relations were absent in infected E. maculata.