Survival of Xanthomonas fragariae on strawberry in summer nurseries in Florida detected by specific primers and nested polymerase chain reaction

Genomic DNA from strain XF1425 of Xanthomonas fragariae was amplified with primers RST2 and RST3 from the hrp-gene region of Xanthomonas campestris pv. vesicatoria. The polymerase chain reaction (PCR) product was sequenced. Four primers were selected at sites unique to X. fragariae, which were identified by comparison with the sequences of PCR products amplified by the same primers from DNA of three strains of X. campestris pv. vesicatoria. Three primers were specific for amplification of DNA from X. fragariae but not from strains of 16 pathovars of X. campestris or nonpathogenic xanthomonads from strawberry. Bacteria were detected at approximately 104 to 105 CFU/ml by a single round of PCR. A nested PCR technique enabled detection to approximately 18 cells. Restriction endonuclease digestion patterns of the PCR product were unique to X. fragariae and confirmed amplification of DNA from the target organism. Bacteria were detected from symptomatic and asymptomatic plant tissue by the nested technique. From strawberry plants inoculated with a rifampicin-resistant strain of X. fragariae and planted in the field in Florida, bacteria were detected by nested PCR and by recovery onto media at 2-week intervals for 92 days after planting. Daughter plants of the inoculated plants were positive for X. fragariae by nested PCR amplification, indicating that X. fragariae survived on plants in summer nurseries in Florida and was disseminated to daughter plants.