COMPARISON OF A DNA HYBRIDIZATION PROBE AND ELISA FOR THE DETECTION OF CLAVIBACTER-MICHIGANENSIS SUBSP SEPEDONICUS IN FIELD-GROWN POTATOES

Clavibacter michiganensis subsp. sepedonicus, the causal agent of bact erial 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 (Russ et 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 les s aggressive C. m. sepedonicus strain, were processed by directly blot ting cut tissue sections on nylon membranes, macerating frozen tissues , and applying xylem fluid collected by centrifugation to nylon membra nes (stems only). The efficiency of detection was significantly influe nced by sampling date, plant part, inoculum dose, and cultivar. The pr obe was compared with an enzyme-linked immunosorbent assay (ELISA) and showed 95-100% agreement when underground Russet Burbank stems inocul ated with 10(9) cfu of aggressive C. m. sepedonicus strain SS43 were d irectly blotted. Although overall detection rates with stem sections w ere 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.