New Assays for Detection of Pseudomonas syringae pv. glycinea in Soybean Seed. E. ALVAREZ, Department of Plant Pathology, Iowa State University, Ames 50011. E. J. BRAUN, and D. C. McGEE, Seed Science Center and Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 79:12-14. Accepted for publication 3 October 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0012.

Two new assays for seedborne Pseudomonas syringae pv. glycinea, the causal agent of bacterial blight in soybeans, were compared with two existing assays on 21 commercial soybean seed lots. For one of the new assays (C), seeds were ground, suspended in saline, and plated on King's B medium amended with 10 µg/ml cephalexin (KBC). Presumptive colonies were confirmed as P. s. glycinea by pathogenicity to soybean seedlings and by agglutination with antiserum. For the other new assay (D), seeds were washed under running tap water, dried and plated on KBC, and presumptive colonies confirmed as P. s. glycinea in the same way as in assay C. For one of the existing assays (A), seeds were soaked in sterile water, the soak solution plated on KBC, and presumptive colonies of P. s. glycinea confirmed by a positive reaction in the levan test and negative reactions in oxidase and esculin hydrolysis tests. For the other existing assay (B), seeds were soaked in a buffer solution, the soak solution centrifuged, the resuspended pellet plated on KBC, and presumptive colonies confirmed as P. s. glycinea by immunofluorescence. Assay C consistently detected higher numbers of colony-forming units of the pathogen than did assays A and B and was completed in 7 days compared with 12 and 15 days for B and A, respectively. Although a different unit of measurement (percentage of infected seeds) and a tenfold smaller number of seeds was used in assay D, results suggested that the assay had a high degree of sensitivity. Rankings for incidence of seedborne P. s. glycinea across the 21 seed lots were strongly correlated (r = 0.50 – 0.97) between all assays. The validity of the nutritional, Serological, and pathological tests that were components of the different assays was confirmed when all of the 121 strains of P. s. glycinea obtained in the four assays gave the correct reaction when submitted to each test.