A New Physiological Race of Phytophthora sojae on Soybean. R. E. Wagner, Department of Plant Pathology, University of Illinois, Urbana 61801. H. T. Wilkinson, Department of Plant Pathology, University of Illinois, Urbana 61801. Plant Dis. 76:212. Accepted for publication 7 August 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0212C.
Twenty-six physiological races of Phytophthora sojae M.J. Kaufman
& J.W. Gerdemann, the causal agent of Phytophthora root and stem
rot of soybean (Glycine max (L.) Merr.) have been reported (1; B. L.
Keeling, personal communication). A new race, designated race 27,
was obtained from a culture isolated from soybean and identified
as race 16 (B. L. Keeling, personal communication). Race 27 originated from a single zoospore after five successive single zoospore transfers.
The identity of race 27 was determined by inoculating taproots or
hypocotyls of differential soybean cultivars incubated at 24 C. Soybean
cultivars Altona, Harosoy, Mack, PI 171442, Sanga, and Williams
and isolines of Williams containing Rps 1-b, Rps 1-c, Rps 1-k, or
Rps 3 were susceptible to race 27, and Harosoy 63, PI 103091, and
an isoline of Williams containing Rps 1 were resistant. Subcultures
originating from single zoospores have been previously reported to
differ in virulence from their parent culture (2).