Biology and management of blackleg disease of potato caused by Dickeya dianthicola (ME23)

Abstract

Potato is the most commonly consumed vegetable in the United States, where people each an average of 49.2 pounds per person per year. About 80% of potatoes in the US are produced in Idaho, followed by Washington, Wisconsin, and Oregon. Potato is a vegetatively propagated crop, and progeny tubers serve as seed for cultivation the following year. Therefore, tuber-borne pathogens, such as bacteria that cause blackleg, result in serious economic losses when progeny tubers are contaminated by pathogens. Blackleg of potato is characterized by blackening of the basal part of stem and rotting of seed tubers. It is caused by Pectobacterium and Dickeya species, which are in the Pectobacteriaceae family and are collectively referred to as the soft rot Pectobacteriaceae (SRP). In 2015, multiples outbreaks of blackleg and soft rot occurred in Northeastern United Sates. This outbreak of the disease also impacted potato production all across the neighboring states, as well as other northeastern and mid-Atlantic states where Maine seed potatoes were shipped. It is most likely that prior to the Dickeya dianthicola (ME23) outbreak in the US in 2015, Dickeya was present in seed potatoes and farms in the affected states for at least a few years. However, rain in 2013 and 2014 spread the pathogen and cool temperatures caused the bacterium to remain latent in the tubers. Warm temperature in 2015 on commercial farms that used this Dickeya-infested seed resulted in significant outbreaks. The fields with outbreaks had no previous history of blackleg, the pathogen was suspected to have been present somewhere in the environment, then multiplied suddenly in response to favorable conditions, such as a heavy rain with subsequent surface pooling, and thus caused an outbreak. To prevent further spread of the disease, the primary infection source and the route of infection of the blackleg pathogen must be identified. Being able to discriminate exact subspecies of Dickeya from the others could help reduce the infection and to understand the epidemiology of the pathogen. Therefore, my research focused on development of reliable and accessible detecting tools for D. dianthicola (ME23). Unfortunately, many commercial potato varieties are susceptible to the diseases caused by SRP. Very few are tolerant, and production is compromised due to infection caused by D. dianthicola and high risk of spreading bacteria in other farms if potato seeds are infected. This led to an urgent need to screen for resistance against blackleg disease. There is insufficient information available for potato breeders on relative resistance or tolerance of commercial potato varieties to Dickeya and Pectobacterium spp. For the purpose of our work with SRP, we use the term resistance for plants that remain asymptomatic, or nearly so, after inoculation with Dickeya or Pectobacterium in typical temperature, humidity, and oxygen-level conditions. In addition, there is almost zero evidence of single gene resistance against this pathogen. Rather, disease resistance is quantitative and multigenetic, making it difficult for plant breeders to select for resistance. In addition, blackleg development is highly sensitive to multiple environmental factors including, plant age, availability of favorable environmental conditions and other bacterial pathogen present in the environment, making it difficult to screen varieties for resistance. The molecular and biochemical mechanisms underlying these quantitative resistances are also poorly understood. Therefore, are not efficiently utilized in potato breeding programs, altogether this makes it difficult to achieve true blackleg disease resistance. Nevertheless, it has been previously reported that plant resistant relies on production of small molecules such as phytoalexins or phytoanticipins associated with core resistant pathways. For example, these pathways may induce plant hormones associated with resistance, or antimicrobial peptides or enhance cell wall modifications as a physical barrier against plant pathogens. Interestingly, some accessions of the wild diploid species of potato (Solanum chacoense) are resistant to blackleg and soft rot diseases caused by SRP. My research focuses on identification of resistant lines of wild diploid potato relatives using physiological, biochemical and metabolic profile. In my work, I found that the metabolic profile of resistant stem extracts of S. chacoense consists of small molecules including phenolics, alkaloids, lipids, amino acids and organic acids, some of which may play a significant role in antimicrobial and anti-virulence properties. I found that the biochemical assays including quorum sensing (QS) and plant cell wall degrading enzymes (PCWDE) correlated with metabolites identified in metabolic profile of resistant accessions. Hence, these assays can be used as a less time consuming and easy tool for screening resistant lines against SRP. From these findings, I hypothesize that QS inhibiting molecules are responsible for triggering resistance against blackleg in S. chacoense and can be used as a potential tool in future breeding programs to achieve maximum resistance in our commercially grown potato varieties.