Submitted to: Acta Horticulturae
Publication Type: Abstract only
Publication Acceptance Date: 1/31/2011
Publication Date: 5/1/2011
Citation: Carey, A., Pusey, P.L., Loper, J.E., Stockwell, V. 2011. Plasmid content of isolates of Erwinia amylovora from orchards in Washington and Oregon in the USA. Acta Horticulturae. 896:123-126. Interpretive Summary:
Technical Abstract: Nearly all strains of Erwinia amylovora carry plasmid pEA29, which has not been found in other species of bacteria. Additional plasmids have been reported in the pathogen isolates from Western states, such as a plasmid in strain CA11 that carries streptomycin-resistance genes and the plasmid pEU30, which was detected in several isolates from this region (Foster et al. 2004. Appl. Environ. Microbiol. 70: 7539–7544). Washington (WA) and Oregon (OR) represent a major pome fruit production region of the USA and streptomycin-resistant isolates of E. amylovora are common in orchards in this region. We examined the plasmid content of a collection of more than 200 isolates of E. amylovora from WA and OR with RFLP and PCR assays. Strains were isolated from infected pear and apple and stored in nutrient broth with 15% glycerol at -80°C. 97% of isolates harbored the plasmid pEA29, but ~3% of the isolates lacked pEA29. The isolates lacking pEA29 were obtained from a few orchards in central WA and one in Northeast OR. To our knowledge, this is the first report of E. amylovora lacking pEA29 in the USA. Interestingly, other isolates of the pathogen from those same orchards harbored pEA29, so the pathogen population within an orchard was not homogeneous in plasmid content. The RFLP patterns of plasmids from ~60% of isolates were similar to one another and reflect the pattern associated with pEA29, but plasmid preparations from about 40% of the isolates had altered RFLP patterns, possibility due to the presence of plasmid(s) in addition to pEA29. The majority of those isolates were positive for pEU30 by a PCR assay. The broad distribution of pEU30 among strains of E. amylovora isolated throughout WA and Northern OR in this study supports the results of Foster et al. About half of the isolates were resistant to streptomycin and there was no correlation between resistance to streptomycin and plasmid content. None of the strains tested carried strA-strB; so resistance to streptomycin likely was due to spontaneous chromosomal mutation. The majority of strains in this region were typical of E. amylovora and harbored only pEA29. Nonetheless, many of the pathogen isolates had altered plasmid content, indicating that plasmid acquisition and propagation in populations of E. amylovora in orchards in the Northwestern USA is more common than previously assumed.