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Effect of Phloem Water Relations on the Growth of Phytophthora cinnamomi in Eucalyptus marginata. Joanna T. Tippett, Department of Conservation & Land Management, 50 Hayman Road, Como, Western Australia 6152, Australia; D. S. Crombie, and T. C. Hill. Department of Conservation & Land Management, 50 Hayman Road, Como, Western Australia 6152, Australia. Phytopathology 77:246-250. Accepted for publication 16 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-246.

The growth rate of Phytophthora cinnamomi in the secondary phloem of Eucalyptus marginata was determined, in part, by the water status of the tissue. Phloem of trees with the greatest water deficits was the least susceptible to invasion by the fungus. The effect of tissue water status on growth of P. cinnamomi was investigated by establishing 10 plots of 15 saplings each in different rainfall zones of the northern jarrah forest in Western Australia. Stem phloem of each sapling was inoculated with P. cinnamomi in early summer, and fungal growth was monitored for 3 mo by means of a Plant Impedance Ratio Meter. Phloem relative water content (RWC) was determined on the same dates that fungal growth was estimated. At some plots, phloem RWCs decreased appreciably as summer progressed. When phloem RWCs were below 85%, lesion extension ceased even though summer temperatures were highly favorable for fungal growth. Mean predawn leaf water potentials (at selected plots) in late summer varied between 0.63 and 2.5 MPa. Excised phloem pieces were used to determine the relationship of RWC to phloem water potential. RWC was related linearly to water potential over the range 75100% RWC, corresponding to water potentials of 1.5 to 0 MPa, respectively. Fungal growth in excised stem blocks was also correlated with phloem water potential and RWC.