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Seedborne Diaporthe phaseolorum var. caulivora in Iowa and Its Relationship to Soybean Stem Canker in the Southern United States. D. C. McGee, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. J. A. Biddle, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 71:620-622. Accepted for publication 17 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0620.

Soybean pods collected from fields in Iowa in 1981 and 1982 were extensively colonized by Diaporthe phaseolorum var. caulivora, the cause of soybean stem canker. Plants with symptoms of stem canker, however, were not found in the fields. Twenty-three isolates of D. phaseolorum var. caulivora from seeds and stems of soybeans grown in different locations in Iowa in 1983 and 16 isolates of D. phaseolorum from stem-cankered plants from Mississippi, Georgia, and Florida were tested for pathogenicity against seedlings of soybean cultivars Bragg, Tracy-M, Harosoy, Hawkeye, Williams 82, and BSR 201 under laboratory conditions. All Iowa isolates were moderately virulent on all six cultivars. Ten southern isolates were highly virulent on Bragg and avirulent on the other cultivars, and six were moderately virulent on Bragg and BSR 201 and avirulent on the others. Cultural tests, made by growing isolates for 5 wk on acid PDA plates at 25 C under constant light, showed that isolates from Iowa and southern states were easily distinguishable by mycelial texture, chlamydospore production, stromatal size, shape, and distribution, presence of pycnidia or perithecia, and thickness of perithecial necks. Iowa isolates were extremely uniform in cultural characters. Southern isolates showed considerable variability in the degree of chlamydospore production but were uniform for other traits.