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Pectate and Pectin Gels for Differentiation of Pseudomonas sp. and Other Bacterial Plant Pathogens. D. C. Hildebrand, Associate Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720; Phytopathology 61:1430-1436. Accepted for publication 6 July 1971. DOI: 10.1094/Phyto-61-1430.

One hundred and sixty-five strains of 54 Pseudomonas sp., 32 strains of 12 Erwinia sp., three Agrobacterium sp., one Corynebacterium sp., and seven Xanthomonas sp. were tested for pit formation on polypectate gels of three different pH ranges (4.9-5.1, 6.9-7.1, and 8.3-8.5). Of the Pseudomonas sp., 92 strains produced pitting at the low pH (medium A), and 29 formed pits at the highest pH (medium C). Only P. solanacearum strains, the angular leaf-spotting pathogens, and one of two strains of P. caryophylli formed pits at all pH levels. Pitting at a given pH was consistent among strains of the same species. The addition of metabolizable substrates to the gels affected pit formation, presumably through a shift in pH. Pitting on pectin gels by most organisms was similar to that on polypectate gels, except that pits were smaller. All Erwinia sp., except E. amylovora, and all Xanthomonas sp. tested also formed pits on the polypectate gels, and, as was found for Pseudomonas sp., the pH levels at which pitting occurred depended upon the species of pathogen. That strains of a species usually were consistent in their abilities to cause pitting on polypectate gels makes this property a useful taxonomic character in the differentiation of various plant pathogens.