First Report of Potato Blackleg Caused by Pectobacterium carotovorum subsp. brasiliense in Switzerland

Blackleg and soft rot are common potato (Solanum tuberosum) diseases found all over the world, causing severe economic damages. In Europe, bacteria like Pectobacterium atrosepticum, P. carotovorum subsp. carotovorum, P. wasabiae, Dickeya solani, and D. dianthicola have been isolated from diseased stems and rotten tubers; they are the causal agents of the disease (5). A survey was made during the 2013 growing season on the Swiss Plateau in the cantons of Bern and Fribourg, the main potato-growing region of Switzerland. Among a sample of 106 diseased plants, 86 were found to be infected by one of the above-mentioned bacterial species, mainly the two Dickeya species and P. wasabiae. Bacterial strains isolated on 16 remaining plants with symptomatic soft rot at the base of the stem exhibited pectinolytic activity on crystal violet pectate agar but failed to be identified by PCR assay as one of these species. The strains were Gram-negative rods, facultatively anaerobic, and grew at 28 and 37°C, and pectinolytic activity was observed on surface-disinfected potato slices (cv. Markies). (No maceration was observed in the negative control with water.) The strain was also positive by PCR assay with pel gene-specific primers Y1/Y2 (1), which are specific for Pectobacterium species. PCR with specific primers BR1f/L1r for the detection of P. carotovorum subsp. brasiliense was also positive (2). Molecular identification of four of these strains (HAFL05 to HAFL08), originating from four different seed lots, was performed by sequencing regions of seven housekeeping genes: acnA, gapA, icdA, mdh, mtlD, pgi, and proA (3). Multilocus sequence analysis was done using concatenated DNA sequences (2,686-bp) of these genes (GenBank Accession Nos. KM017537 to KM017552 and KP027692 to KP027735) and sequences previously deposited in GenBank. Clustered strains HAFL05 to HAFL08 shared identity with each other (100%) and with other P. c. subsp. brasiliense strains: IPO-3650 (99.9% identity), strain 1009 (98.4%), Ecbr1692 (97.3%), and NZEC1 (97.2%). Compared with the most closely related strain, P. c. subsp. carotovorum CFBP 2046, the P. c. subsp. brasiliense Swiss strains shared 95.6% identity. Moreover, characterization by whole-cell MALDI-TOF mass spectrometry clustered the Swiss strains close to the reference strain P. c. subsp. brasiliense Ecbr1692. In a growth chamber experiment, 5-week-old plantlets of cv. Markies were inoculated by injecting 100 μl of a 10⁶ CFU/ml solution of the P. c. subsp. brasiliense strain at the base of the stem. Plants were incubated in a growth chamber at 25°C and a relative humidity of 80%, with regular watering. Stem rot (brown water-soaked and necrotic lesions) appeared after 20 days, whereas water-inoculated plantlets remained symptomless. Strains obtained from the artificially diseased stems were confirmed as P. c. subsp. brasiliense using specific primers BR1f/L1r (2), confirming the strain’s pathogenicity. P. c. subsp. brasiliense has already been described in Brazil in 2004 (2), and then in South Africa and Canada. In Europe, the first report of P. c. subsp. brasiliense was recently published for the Netherlands (4). This is the first report of P. c. subsp. brasiliense as causal agent of blackleg in Switzerland. Its further spread should be considered seriously, as during the 2014 growing season, on a sample of 100 diseased plants from the same region, P. c. subsp. brasiliense was responsible for about 70% of the blackleg incidence.