THE DEVELOPMENT OF A CONDUCTIMETRIC ASSAY FOR AUTOMATED DETECTION OF METABOLICALLY ACTIVE SOFT-ROT ERWINIA SPP IN POTATO-TUBER PEEL EXTRACTS

Four media were tested for their ability to detect the soft rot potato pathogens Erwinia chrysanthemi (Ech) and Erwinia carotovora ssp. atro septica (Eca) in potato tubers by means of automated conductance measu rements. The specificity of the conductimetric assays was determined b y testing a set of different Erwinia spp. and potato-associated saprop hytes, including the genera Pseudomonas, Bacillus, Enterobacter and Fl avobacterium, All bacteria tested produced conductance responses in Sp ecial Peptone Yeast Extract, whereas in minimal medium with L-asparagi ne only Erwinia spp. and Pseudomonas spp. were able to generate large conductance responses. In minimal medium supplemented with glucose and trimethylamine-N-oxide only Enterobacteriaceae, Erwinia spp. included , generated conductance responses, while with pectate as sole carbon s ource only Erwinia spp. produced distinct conductance responses. The p ectate medium proved to be particularly useful for specific automated conductimetric detection of Erwinia spp. in potato peel extracts. With in 48 h, the detection threshold of the conductimetric assay for Eca v aried between 10(2) and 10(3) cfu per mi peel extract at both incubati on temperatures of 20 degrees and 26 degrees C. Ech was detected at co ncentrations of 10(4)-10(5) or 10(3)-10(4) cfu ml(-1) at 20 degrees an d 26 degrees C, respectively. To eliminate 'false'-positive reactions in conductimetry caused by Erwinia carotovora ssp. carotovora, results of the conductance measurements have to be confirmed by other techniq ues, like serology or DNA assays.