Microbiological, immunological and molecular methods suitable for commercial detection and quantification of the blackleg pathogen, Erwinia carotovora subsp. atroseptica, on seed potato tubers: a review

Contamination of seed potato tubers by Erwinia carotovora subsp. atroseptica is widespread with the bacteria usually sited superficially in lenticels and suberized wounds. As seed contamination level is related to blackleg incidence, seed health is best assessed by determining the number of cells of E. c. atroseptica per mL of tuber-peel extract. The relative specificity, sensitivity and ease of use of four recently developed microbiological, immunological and molecular methods to detect and/or quantify tuber contamination are discussed in relation to the testing of commercial seed stock. Sensitivities of all four methods are at or below the threshold level for blackleg development (< 10³ cells mL^-1), but there are differences regarding their specificity and ease of use. Three of them allow enumeration of most live cells of the bacterium, using specific monoclonal and polyclonal antibodies against the predominant serogroup I: (a) immunomagnetic separation of E. c. atroseptica before viable count on a selective-diagnostic growth medium, crystal violet pectate, (b) immunofluorescence staining and counting of colonies in pour-plate medium in tissue culture plates and (c) enrichment of the bacterium in peel-extract dilutions directly in microtitre plates prior to DAS-ELISA. In the fourth method, both live and dead cells are detected, but not quantified, by PCR amplification of target sequences using specific primers for E. c. atroseptica regardless of serogroup.