This article appears in the April 2020 issue of Potato Grower.
In 2015, there was an aggressive outbreak of blackleg on potato in the eastern half of the United States. This outbreak was caused by the bacterial pathogens Dickeya dianthicola, and it led to large annual losses for potato growers.
Although there are multiple species of Dickeya that can cause potato disease, the only one that appears to be the cause of the North American outbreak is D. dianthicola. Multiple seed potato-producing states were tied to the outbreak, and multiple types of D. dianthicola were found, suggesting that this pathogen had been present in North American potato production for at least a few years before the outbreak was recognized.
Dickeya causes blackleg symptoms very similar to those caused by the more familiar and widespread Pectobacterium. Although it is an aggressive blackleg pathogen, it is an ineffective soft rot pathogen and does not tend to cause tuber decay. However, it can cause disease along with Pectobacterium, which effectively decays tubers. Together, these two pathogens can cause more loss than either one alone.
Dickeya survives well in and on potato tubers—one of the ways it is able to spread from farm to farm. Fortunately, Dickeya does not appear to survive well in soil, so unlike some pathogens, it will not become established in farm fields if farmers use even minimal two- or three-year rotations and manage volunteer potatoes. However, this pathogen can become established in surface water, including in irrigation ponds and streams.
One of the most important things growers can do to manage Dickeya is to test seed potatoes for the pathogen prior to planting the seed. Researchers in Scotland developed useful PCR tests for D. dianthicola, but only one of the two PCR assays is able to detect all of the strains found in North American potatoes. Unlike some bacterial potato pathogens, such as Clavibacter (the cause of bacterial ring rot), Dickeya and Pectobacterium have highly variable genomes. This means that relying on just one or a few PCR tests will cause growers to miss some pathogen strains.
Recently, a user-friendly online computer program, Uniqprimer, was developed at Colorado State University. This computer program can use bacterial genome sequences to quickly and automatically design PCR assays for pathogens. Researchers used Uniqprimer with multiple Dickeya genome sequences and sequences from related pathogens to develop two new PCR tests, then validated those tests with potato samples from California, Florida, Michigan, Missouri, New Mexico, New York, North Carolina, Texas and Wisconsin. This brings the number of validated PCR tests for D. dianthicola that work with North American strains from a single test to three tests.
We believe that Uniqprimer will have a high impact on agriculture because this program can quickly design PCR tests, it is freely available to anyone, and it requires only basic computer operating skills.
Some U.S. potato farmers have made testing for Dickeya a routine part of their seed potato health program. They believe this has helped them avoid purchasing highly infested seed lots, helping them assess risk for their farms. Interpreting test results is still a challenge because, like many potato pathogens, the test only gives the grower an estimate of risk. Whether or not disease develops and how severe losses will be depends in part on environmental conditions for that year. However, a risk estimate still aids growers in making decisions about seed lot purchases and how their fields are managed, so this testing is a necessary part of seed potato health management.
We are currently using Uniqprimer to develop PCR tests for Pectobacterium strains common in North America. For example, we are currently validating a PCR tested for Pectobacterium parmentieri, an aggressive soft rot pathogen that is often found alongside Dickeya in potatoes. These PCR tests will aid growers in managing disease and will aid researchers in determining how these pathogens enter seed potato systems and spread among farms. With development of these assays, U.S. growers will finally be able to use tests to determine exactly which species are causing blackleg and soft rot on their farms, which is the first step in finally being able to effectively manage this common scourge of potato production.
Shaista Karim is a graduate student at Colorado State University’s College of Agricultural Sciences, pursuing a Ph.D. in plant pathology. She can be contacted at shaista.karim@colostate.edu