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Clavibacter michiganensis subsp. sepedonicus, the causal agent of bacterial ring rot, was detected in field-grown potatoes using a 1.078-kb repeated C. m. sepedonicus sequence as a probe in DNA hybridizations. Stem and petiole samples from susceptible and tolerant cultivars (Russet Burbank and Belrus, respectively), inoculated with 10 mM phosphate buffer (pH 7.2) or 102 or 109 cfu of either an aggressive or a less aggressive C. m. sepedonicus strain, were processed by directly blotting cut tissue sections on nylon membranes, macerating frozen tissues, and applying xylem fluid collected by centrifugation to nylon membranes (stems only). The efficiency of detection was significantly influenced by sampling date, plant part, inoculum dose, and cultivar. The probe was compared with an enzyme-linked immunosorbent assay (ELISA) and showed 95-100% agreement when underground Russet Burbank stems inoculated with 109 cfu of aggressive C. m. sepedonicus strain SS43 were directly blotted. Although overall detection rates with stem sections were higher for ELISA (18.4% with ELISA vs. 11.3% with direct blotting), a high rate of false positives (53.9%) occurred with petiole tissues at 90 days after planting when ELISA was used, whereas none occurred with DNA hybridizations.