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Population Dynamics and Pathogenesis of Pseudomonas syringae in Maize and Cowpea in Relation to the In Vitro Production of Syringomycin. D. C. Gross, Department of Plant Pathology, University of California, Davis, CA 95616, Present address of senior author: Department of Plant Pathology, University of Nebraska, Lincoln, NB 68503; J. E. DeVay, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 67:475-483. Accepted for publication 15 November 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-475.

Fifteen species of Pseudomonas were compared for pathogenicity in maize and cowpea and for synthesis of syringomycin (SR) in agar and broth media. Fifty-five out of 75 isolates of P. syringae from various hosts produced SR; some isolates produced up to 3,300 units SR/ml of potato-dextrose broth (PDB). Twenty-eight representative strains of other Pseudomonas spp. (primarily plant pathogens) did not produce SR. Partial vacuum inoculation (108 cells/ml) of all Pseudomonas isolates into maize delineated a marked specificity of grass isolates of P. syringae for maize. Maize leaves were infiltrated with either of two isolates of P. syringae, pathogenic on maize, with inocula concentrations to give initial populations of 2 102 and 2 105 bacteria/0.32 cm2 leaf disk; each isolate grew exponentially for 24-48 hr and attained final populations of 5 106 and 3 107 cells/leaf disk corresponding to the low and high inoculum concentrations, respectively. The end of the exponential growth phase of the bacteria in leaf tissue marked a cessation of lesion development. Growth of nonpathogenic isolates of P. syringae in maize tissue, including both producers and nonproducers of SR, was arrested with the development of a hypersensitive reaction (HR). All isolates that caused holcus spot on maize produced significant amounts of SR in PDB. Maize was at least twice as sensitive to SR than was the standard bioassay fungus, Geotrichum candidum. There was a strong relationship between systemic necrosis (SN) in inoculated cowpea hypocotyls and SR production by P. syringae. Systemic necrosis was caused by most SR-producing (in vitro) isolates, but not by any of the SR-negative isolates. However, population studies with SN-positive and SN-negative isolates of Pseudomonas spp. did not show large differences in multiplication in host tissues.