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Identity and Relative Virulence of Some Heterothallic Phytophthora Species Associated with Root and Stem Rot of Safflower. J. M. Klisiewicz, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis, CA 95616; Phytopathology 67:1174-1177. Accepted for publication 19 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1174.

Phytophthora species isolated from safflower plants affected with root and/or stem rot in production areas in Arizona and California were identified as P. cryptogea, P. drechsleri, and P. parasitica. Only P. drechsleri previously had been implicated as a causal organism of root rot in nature. Isolates of each species induced root and stem rot of artificially inoculated plants of susceptible cultivar Nebraska 10 and moderately resistant cultivar Gila. Most isolates did not induce root or stem rot in resistant cultivars VFR 1 and US Biggs. Highly virulent isolates of P. cryptogea and P. drechsleri induced root rot of root-rot-resistant VFR 1 and stem rot of stem-rot-resistant US Biggs.