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Serology and Pathology of Pseudomonas syringae. J. D. Otta, Former Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Department of Plant Science, South Dakota State University, Brookings 57006; Harley English, Professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 61:443-452. Accepted for publication 24 September 1970. DOI: 10.1094/Phyto-61-443.

Approximately 450 isolates of bacteria classified as Pseudomonas syringae, from more than 30 host plants, were received from around the world. Each isolate was studied for oxidase reaction, pathogenicity to peach seedlings, serological reaction, and antibiotic production as measured by inhibition of Geotrichum candidum. Sixty isolates were oxidase-positive, nonpathogenic on peach seedlings, and serologically distinct from pathogenic isolates (and hence considered misclassified). Most oxidase-negative isolates were confirmed as P. syringae by their pathogenicity to peach seedlings, and it is concluded that peach seedling pathogenicity can be used to detect P. syringae isolates originating from many hosts of this organism. Peach seedling pathogenicity was not correlated with the ability to inhibit G. candidum. The pathogenic isolates were divided into 10 distinct serotypes based on the reaction of their heat-stable antigens in gel-diffusion tests. Host of origin was generally not correlated with serotype. Composite antigens containing sonicated cells of seven of the major P. syringae serotypes were used to obtain composite antisera containing antibodies against the heat-stable antigens of each serotype. Rough and smooth isolates possessed serologically identical heat-stable antigens, were not consistently different pathologically, and did not agglutinate in up to 10% saline solutions. Isolates received as P. aptata and P. morsprunorum were similar enough to P. syringae in all tests to be considered synonymous with it.