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Disease Note.

Two New Races of Phytophthora sojae, Causal Agent of Phytophthora Root and Stem Rot of Soybean, Identified from Arkansas Soybean Fields. R. N. Henry, Southwest Research and Extension Center, 362 Highway 174 N, University of Arkansas, Hope 71801.. T. L. Kirkpatrick, Southwest Research and Extension Center, 362 Highway 174 N, University of Arkansas, Hope 71801. Plant Dis. 79:1074. Accepted for publication 28 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1074B.

Isolates of Phytophthora sojae M. J. Kaufman and J. W. Gerdemann were collected in April and May of 1994 from soil of Arkansas soybean {Glycine max L. Merr.) fields during a survey to determine incidence and predominant races. Soil samples were collected from 54 random fields in 23 counties and were evaluated using a seedling bio-assay developed by Schmitthenner (1). Soybean cultivar Sloan was used as a bait for P. sojae. Single-zoospore cultures, selected from isolates of the pathogen recovered in the bio-assay, were screened for race using the hypocotyl-wound technique. Cultivars were Harlon (Rps 1), Harosoy 13xx (Rps l-b), Williams 79 (Rps 1-c), P. I. 103.091 (Rps 1-d), Williams 82 (Rps 1-k), L83-570 (Rps 3), Harosoy 62xx (Rps 6), and Harosoy (Rps 7). The virulence evaluation technique (1) was modified by substituting dilute lima bean agar for V8 juice agar and using 400 watt metal halide lights instead of fluorescent lights. An isolate collected from Chicot County in southeastern Arkansas was pathogenic on each of the eight differential cultivars. This test was repeated five times with consistent results. Another isolate from Woodruff County was pathogenic on every differential cultivar except P. I. 103.091. The differential test for this isolate was repeated three times with the same result. Neither of these virulence patterns has been previously reported for field-collected isolates (1,2). We propose these as races 38 and 39, respectively.

References: (1) A. F. Schmitthenner et al. Plant Dis. 78:269, 1994. (2) H. Forster et al Mol. Plant-Microbe Interact. 7:780, 1994.