Pectobacterium aroidearum and Pectobacterium carotovorum subsp. carotovorum as causal agents of potato soft rot in Lebanon

To identify bacteria causing severe soft rot on potato in Lebanon, pectinolytic bacteria were isolated from diseased samples collected in the Bekaa valley and the Akkar region, during 2009–2010. Six isolates that induced a hypersensitive reaction in tobacco plants and caused a marked soft rot when inoculated in potato tubers or stem, DAPP-PG 752 to DAPP-PG 757, were further characterized. Analysis of 16S–23S intergenic transcribed spacer-regions, digested with RsaI and CfoI, demonstrated that isolates DAPP-PG 753 to DAPP-PG 757 showed electrophoretic profiles similar to that of LMG 2408, a strain misclassified as Pectobacterium carotovorum subsp. carotovorum, and different from that of DAPP-PG 752. Phylogenetic analysis using partial malate dehydrogenase gene sequences, clustered DAPP-PG 752 in a clade that contained the type strain of P. carotovorum subsp. carotovorum, while DAPP-PG 753 to DAPP-PG 757, together with LMG 2408, fell in a clade including strains of the recently described species Pectobacterium aroidearum. Multilocus sequence analysis of concatenated partial sequences of the atpD, gyrB, infB and rpoB genes confirmed the results obtained with the mdh gene sequences and identified DAPP-PG 752 as P. carotovorum, and LMG 2408 and the five related Lebanese isolates as P. aroidearum. The Lebanese P. aroidearum isolates were phenotypically similar to known P. aroidearum strains, based on API 50 CHE data. They showed identical BOX- and ERIC-PCR profiles that resembled profiles of known P. aroidearum strains. This study represents the first official report of P. aroidearum as causal agent of soft rot on potato plants and the first occurrence of P. carotovorum subsp. carotovorum on potato plants in Lebanon.