- Bacterial soft rot of potato (Solanum tuberosum), caused by Pectobacterium and Dickeya species, is among the most common and destructive potato diseases in the United States. These pathogens cause a variety of vascular wilts, and in potato cause a disease complex that includes tuber soft rot, blackleg, aerial stem rot, and lenticel rot. The Columbia Basin of Northeastern Oregon and Southwestern Washington is a valuable potato producing region. Two projects were conducted at the Hermiston Agricultural Research and Extension Center in Hermiston, Oregon, to address knowledge gaps surrounding soft rot of potato.
The distribution and identities of the multiple bacterial pathogens that cause soft rot of potato has not been recently characterized in the Columbia Basin. The first project was conducted to characterize the genetic diversity of soft rot pathogens of potato in the Columbia Basin. In 2018 and 2019, 25 and 120 diseased plant samples, respectively, that exhibited symptoms of soft rot and originated from the Columbia Basin were received and analyzed using diagnostic PCR assays to identify the isolates. Fifty-four soft rot pathogens were detected in 51 of the samples, that included P. carotovorum (61.1%), P. atrosepticum (20.4%), Dickeya species (5.6%), P. parmentieri (9.3%), and P. brasiliense (3.7%). Twenty-eight bacterial isolates were obtained in culture, although no Dickeya spp. were recovered. The identity of these isolates was confirmed through a phylogenetic assessment of dnaX, pelY, and 16s rRNA sequences. We found that in 2018 and 2019, the soft rot pathogens of potato that were present in the Columbia Basin were P. carotovorum, P. atrosepticum, P. brasiliense, and P. parmentieri. We concluded that although Dickeya spp. may be present, Pectobacterium spp. were the dominant pathogens associated with soft rot of potato in the Columbia Basin in 2018 and 2019.
Potato yield loss associated with seed-borne infections of soft rot pathogens has not been recently estimated. The second project was conducted to determine the yield losses associated with seed-borne soft rot infections to determine how potato yields vary as a function of inoculum prevalence in seed potato in Eastern Oregon. In 2018, potato seed of ‘Lamoka’ and ‘Russet Burbank’ cultivars were inoculated with Pectobacterium carotovorum and Dickeya chrysanthemi and in 2019, potato seed of ‘Lamoka’ and ‘Russet Burbank’ cultivars were inoculated with Pectobacterium atrosepticum and Pectobacterium parmentieri. In each year, inoculated and non-inoculated seed potato was mixed to create planting stock with 0%, 5%, 10%, 20%, and 30% incidence of soft rot. The resulting 20 treatments (2 cultivars x 2 strains x 5 doses of inoculum) were planted in the field and managed using grower practices typical for the region. Emergence, plant health, and blackleg incidence was monitored throughout the growing season. Potatoes from each plot were harvested, graded, and total yield for each plot was calculated by weight. We observed that an increase of bacterial inoculum in the potato seed lead to lower emergence rates for the ‘Lamoka’ cultivar treatments in 2018. Lower rates of emergence lead to lower yields. We did not observe effects of the treatments in either year on plant health. Based on the results of this study, in Eastern Oregon, a 0- 30% incidence of soft rot bacteria in ‘Russet Burbank’ cultivar potato seed does not contribute to lowered emergence, plant health, or yield. A 5 - 30% incidence of soft rot bacteria in ‘Lamoka’ seed may impact plant emergence and yield.