Studies on the genetic diversity of the potato brown rot pathogen Ralstonia solanacearum race 3/biovar 2A
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Ralstonia solanacearum is a genetically diverse and geographically widespread plant pathogen. It has a wide host range and is a significant pathogen of potato, causing brown rot. Brown rot is caused by a distinct, closely-related, intraspecific group: race 3, biovar 2A. In Europe, infection of potato crops with brown rot primarily occurs via irrigation with contaminated surface water. Brown rot has never been found in Scottish potatoes though the bacterium has been found previously in one Scottish river system, the River Tay, both in water samples and on its secondary host, bittersweet (Solanum dulcamara), growing on the river banks. A molecular strain-typing method principally used in clinical microbiology, multi-locus sequence typing (MLST), was used to study genetic variation within a collection of 106 R. solanacearum isolates, principally race 3 biovar 2A isolates from potato, S. dulcamara and contaminated water sources. Twenty-seven isolates from contaminated water and S. dulcamara from Scotland and other isolates from diverse geographic locations, from a variety of diseased plants and the environment, were resolved into 16 sequence types. A subsequent follow-up to the first experiment was carried out by looking at more variable genes within the race 3, biovar 2A genome and again similar or identical relationships were uncovered. All Scottish isolates were found to be identical and similar to most race 3 biovar 2A isolates tested. Analysis of Variable Number Tandem Repeats (VNTRs) confirmed this observation. After sequencing one of the tandem repeat regions, the results strongly suggest that contamination of the River Tay in Scotland occurred as a single or limited event, since the Scottish isolates had a unique tandem repeat pattern different from the patterns observed for the rest of the race 3 biovar 2A isolates and strains studied. This suggests that the Scottish isolates are clonal and the contamination is a single event and not a multiple contamination.