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Differentiation of Pseudomonas syringae pv. tomato from P. s. syringae with a DNA Hybridization Probe. Timothy P. Denny, Department of Plant Pathology, University of Georgia, Athens 30602; Phytopathology 78:1186-1193. Accepted for publication 8 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1186.

A DNA hybridization probe was developed that hybridized strongly to Pseudomonas syringae pv. tomato DNA but not to P. s. syringae DNA on Southern blots and dot blots. This “PST-DNA probe” consisted of cloned 3.5 and 3.6 kilobase EcoRI fragments of P. s. tomato that were 32P-labeled. The PST-DNA probe detection limits were 2 ng of purified P. s. tomato DNA and DNA released in situ from 1 × 105 cells of P. s. tomato. Similar amounts of P. s. syringae DNA retained about 4% as much of the PST-DNA probe. Sixty-eight strains of P. s. syringae, isolated from a variety of plants including tomato, were not recognized by the PST-DNA probe. The PST-DNA probe also did not hybridize to selected strains of P. aeruginosa, P. cichorii, P. fluorescens, P. viridiflava, or Xanthomonas campestris pv. vesicatoria. The PST-DNA probe hybridized to one or more strains of half of the 20 additional pathovars of P. syringae examined. Cells released from individual P. s. tomato lesions were reliably detected by the PST-DNA probe, even though the amount of DNA recovered varied. A method was developed for using the PST-DNA probe when the amount of DNA on a blot is unknown, and a preliminary test indicated that P. s. tomato and P. s. syringae lesions could be differentiated. The PST-DNA probe has the potential to give definitive results within one day after a fresh tissue sample is brought to the laboratory.