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An Improved Agar Plating Assay for Detecting Pseudomonas syringae pv. syringae and P. s. pv. phaseolicola in Contaminated Bean Seed. S. K. Mohan, Department of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow 83843, Present address: University of Idaho, Research and Extension Center, Aberdeen 83210; N. W. Schaad, Department of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow 83843. Phytopathology 77:1390-1395. Accepted for publication 30 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1390.

An assay employing semiselective agar media was developed for detection of Pseudomonas syringae pv. syringae and P. s. pv. phaseolicola in bean seed. The bacteria were extracted from seed by soaking a 1-kg sample in 3 L of sterile saline with 0.01% Tween 20 for 20 hr at 5 C. P. s. pv. syringae and P. s. pv. phaseolicola were detected by plating 0.1-ml samples of the extract onto triplicate plates of the semiselective agar media and incubating them at room temperature. Two semiselective media were developed: KBC for P. s. pv. syringae and MSP for P. s. pv. phaseolicola. Medium KBC, a modification of medium B of King et al (KB) was made by adding boric acid, cephalexin, and cycloheximide to KB. Recovery of 10 strains of P. s. pv. syringae on KBC ranged from 80 to 139%, and more than 90% of seed-associated saprophytes were inhibited on KBC when compared with KB. As few as 2.1 × 10⁴ colony-forming units (cfu) of P. s. pv. syringae (from infected bean leaf material) added to a 1-kg seed sample were detected on KBC. Medium MSP was made by adding cephalexin, vancomycin, cycloheximide, and bromthymol blue to sucrose peptone agar. MSP showed plating efficiencies of 87–124% for nine strains of P. s. pv. phaseolicola, and more than 80% of bean seed-associated saprophytes growing on KB were inhibited. P. s. pv. phaseolicola was detectable in seed samples containing as few as two naturally contaminated seeds (10²–10⁵ cfu per seed) when plated on MSP. Gravity culled and milled commercial seeds from 14 fields suspected of being infected with P. s. pv. phaseolicola and/or P. s. pv. syringae in 1984 were assayed on the two semiselective media and KB; either or both of the pathogens were detected in all 14 lots on KBC or MSP but in only seven lots on KB.