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Changes in Resistance to Water Transport in Safflower During the Development of Phytophthora Root Rot. J. M. Duniway, Department of Plant Pathology, University of California, Davis, CA 95616; Phytopathology 67:331-337. Accepted for publication 17 September 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-331.

Resistances to the movement of liquid water through healthy safflower plants and safflower plants infected with Phytophthora cryptogea were measured at various times after root inoculation. Healthy plants had a total resistance of 7 105 bars sec cm 1 and 44, 13, and 43% of the resistance in healthy plants was located in the roots, stems, and leaves, respectively. During the development of Phytophthora root rot in safflower, the resistance of both the roots and stems increased progressively, but the resistance to water movement in leaves remained unchanged. By the time infected plants wilted, 5 days after inoculation, resistances of infected roots and stems were more than 8 and 40 times the respective resistances in healthy plants. Measurements of transpiration rate and leaf water potential in intact plants showed that increased root and stem resistances to water movement accounted for the depressions of water potential and wilt symptoms that occurred in infected plants. At all stages of disease development, the influence of infection on root resistance greatly exceeded the influence of infection on the size of root systems. Increases in the xylem resistance of infected plants extended up the stem into internodes from which the pathogen was not isolated.

Additional keywords: water relations, Phytophthora drechsleri, Carthamus tinctorius.