It has been nearly six years since Dickeya dianthicola was first reported in potato in New Jersey in 2015 and many other states up and down the East Coast in the spring and summer of 2016. Before then, this seed-borne pathogen had not been detected in potato fields in the mid-Atlantic region and elsewhere. Unfortunately, some potato growers suffered substantial economic losses during the 2015 and 2016 growing seasons. Organic potato producers who grew very small acreage were also affected by Dickeya dianthicola. Most of the commercial potato acreage in New Jersey and elsewhere was being planted with seed purchased from Maine or Canada. When a disease such as this is so widespread when it first occurs it suggests that contaminated seed is the likely inoculum source. Extension personnel from the region learned from visiting farms and talking with growers that occurrences were associated with specific seed lots. With knowledge of the probable origin of the pathogen, Extension personnel from the region developed best management guidelines for Dickeya dianthicola to help potato growers in the region minimize the potential for a Dickeya outbreak in their operation.
Since that time, along with Dickeya dianthicola, other seed-borne tuber rotting pathogens (Pectobacterium spp.) have routinely been found causing significant problems for potato growers in the region. Research on Dickeya and Pectobacterium has been ongoing in the US and other parts of the world where these pathogens occur with data and results related to the most recent outbreaks being published most recently. In a survey of soft rot bacteria collected from potato fields in New York state during the 2016 growing season, a majority of isolates collected were designated as D. dianthicola or P. parmentieri. Based on their dnaX sequence analysis, the authors determined that the D. dianthicola isolated from potato plants in New York formed a single clade, being genetically identical to each other and to D. dianthicola ME23 isolated in Maine in 2015 (Ma et al., 2018). More recent research by Ge et al (2020, Plant Dis. First Look) surveyed commercial potato fields in Maine as well as suspect Dickeya samples collected from potato seed pieces, tubers, or plants from potato fields in 11 other states from 2015 to 2019. A total of 1183 samples were collected. A total of 256 Dickeya dianthicola isolates were used to identify pathogen genotype (I, II, or III) and the “inoculum geography”. Of these, 231 (~90%) were Type I, 14 (~5%) were Type II, and 11 (~4%) were Type III. In Maine alone, 95% of the total isolates collected from commercial potato fields were Type I. “As such, it was suspected that the original contamination in other states initiated from Maine” since “Maine is the primary seed potato supplier to states in the Northeastern U.S.”. The only consistent genotype found in each year of the study from all states sampled from was Type I for which the authors hypothesized was “likely associated with Maine seed origination”. Not finding Dickeya dianthicola Type II and III in Maine in each year of the study may reflect the fact these types were rare compared to Type I thus a larger sample size was needed to confirm they likely were not present those years. Additionally, while most occurrences of Dickeya dianthicola in potato production fields were associated with seed originated from Maine, there were occurrences associated with seed from Wisconsin and Canada. It is possible Type II and III are principally associated with those seed. Seed source was not determined for the samples.
