Physiologic Specialization of Phialophora gregata on Soybean. D. B. Willmot, Graduate Research Fellow, Department of Agronomy, University of Illinois, Urbana 61801. C. D. Nickell, and L. E. Gray. Professor, Department of Agronomy, and Research Plant Pathologist, USDA-ARS, Department of Plant Pathology, University of Illinois, Urbana 61801. Plant Dis. 73:290-294. Accepted for publication 31 October 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/PD-73-0290.

Isolates of Phialophora gregata were obtained from soybean cultivars BSR 201, PI 437833, Elgin, and Hodgson 78 grown in Illinois, Iowa, Wisconsin, and Minnesota. The isolates were used to inoculate their source cultivars, then reisolated. These four cultivars were used as a differential host set in greenhouse root-dip assays to determine variability among isolates for virulence and aggressiveness, and the experiment was repeated in time. All 27 isolates chosen for detailed study caused defoliation on one or more cultivar. The normally susceptible cultivars Elgin and Hodgson 78 were susceptible to few, but different, isolates. BSR 201 was resistant to all isolates, but PI 437833 was susceptible to four isolates. Isolates derived from BSR 201 were less aggressive on Elgin and Hodgson 78 than were isolates derived from other cultivars. Although the differential host set employed in this study is not conducive for the designation of pathogen races, these data demonstrate a high level of variability for aggressiveness and the first report of physiologic specialization in P. gregata.