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A New High-Temperature Phytophthora Pathogenic to Roots of Alfalfa. O. K. Ribeiro, Postgraduate Research Associate, Department of Plant Pathology, University of California, Riverside, CA 92521; D. C. Erwin(2), and R. A. Khan(3). (2)(3)Professor, and Staff Research Associate, respectively, Department of Plant Pathology, University of California, Riverside, CA 92521. Phytopathology 68:155-161. Accepted for publication 15 July 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-155.

A Phytophthora sp. was isolated from alfalfa (Medicago sativa) with root rot in the Imperial Valley of Southern California during the summer. The pathogen grew optimally at 33-36 C and actively at 36 and 39 C but not at 42 C. Severe root rot under experimental conditions was obtained only at temperatures of 29-32 C and above. Inoculated plants incubated at greenhouse temperatures between 24-27 C were stunted but did not succumb to the pathogen. Although the fungus did not produce oospores under any experimental treatment tested, it was classified on the basis of the morphology of sporangia and hyphal swellings as a high-temperature variant of Phytophthora megasperma and designated as cultivar HTI (high-temperature isolate).

Additional keywords: root rot, proliferation of sporangia, zoospores.