First Report of Pectobacterium wasabiae Causing Aerial Stem Rot of Potato in Washington State

Aerial stem rot of potato (Solanum tuberosum), also known as bacterial stem rot, is often caused by the pectolytic bacteria Pectobacterium (Erwinia) carotovorum subsp. carotovorum, P. atrosepticum, or Dickeya spp. (3). A survey was carried out in August 2008 in 'Russet Burbank' potato fields exhibiting aerial stem rot symptoms in the Columbia Basin of Washington State. One bacterial strain isolated during the survey, PwO405, exhibited pectolytic ability on crystal violet pectate (CVP) agar and potato slices and failed to grow at 37°C, but physiological tests did not conclusively distinguish the bacterium as P. atrosepticum (1). The bacterium was positive for ONPG, N-acetylglucosaminyl transferase, gelatin liquefaction, and acid production from D-galactose, lactose, melibiose, raffinose, citrate, and trehalose. The bacterium was negative for indole production and acid production from maltose, α-methyl-D-glucoside, sorbitol, D-arabitol, inositol, inulin, and melezitose. Molecular identification of the bacterium was performed with 16S rRNA, aconitase (acnA), and malate dehydrogenase (mdh) coding sequences as previously described (2,4). Partial sequences of 16S rRNA (1,408 bp) and acnA (412 bp) genes (GenBank Accession Nos. JQ723958 and JQ723959, respectively) exhibited 99% shared identities with P. wasabiae strain WPP163, while the mdh sequence (435 bp) (GenBank Accession No. JQ723960) exhibited 100% shared identity with mdh sequences from three P. wasabiae strains (NZEC9, NZEC10, and NZEC8974). Maximum parsimony analysis using concatenated acnA and mdh sequences from this study and Pectobacterium sequences previously deposited in GenBank (2,4) clustered strain PwO405 with other P. wasabiae strains. Three 7-week-old 'Russet Norkotah' potato plants were wound-inoculated by inserting a sterile 23 gauge needle just above a central leaf axil at a depth of 1 mm. A 10-μl drop of inoculum (10⁴ CFU) was placed on the wound. Plants were exposed to a 24-h leaf wetness period (90 to 100% RH in a mist chamber) and lesions were measured. All three inoculated plants exhibited aerial stem rot symptoms similar to those observed in the field, including brown water-soaked lesions that spread acropetally and basipetally. Upon drying, the lesions became shriveled and turned dark brown to black. Some plants exhibited hollowing of the stems and unilateral wilt on the side of the lesion. Symptoms were not observed on water-inoculated controls. The bacteria that were reisolated into pure culture from all three inoculated stems caused pitting on CVP and exhibited the same morphology as the original culture and were confirmed as P. wasabiae using 16S rRNA, acnA, and mdh coding sequences, fulfilling Koch's postulates. Stem rot ability of the bacterium was also confirmed on four potato cultivars: 'Ranger Russet,' 'Russet Burbank,' 'Russet Norkotah,' and 'Umatilla Russet' by wound-inoculating six single-stem plants of each cultivar as described above. To our knowledge, this is the first report of aerial stem rot of potato caused by P. wasabiae in Washington State. References: (1) S. De Boer and A. Kelman. Page 56 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria, 3rd ed. N. Schaad et al., ed. APS Press, St. Paul, 2001. (2) A. Pitman et al. Eur. J. Plant Pathol. 32:211, 2010. (3) M. Powelson and G. Franc. Page 10 in: Compendium of Potato Diseases. W. Stevenson et al., ed. APS Press, St. Paul, 2002. (4) M. Yap et al. Appl. Environ. Microbiol. 70:3013, 2004.