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 10 2 or 10 9 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)