Prevalence and genetic diversity of Pseudomonas syringae pv. aptata in table beet and Swiss chard seed production in Washington State

The maritime region of the Pacific Northwest (PNW), encompassing western Oregon and western Washington, is the only region in the United States with suitable environmental conditions for production of beet and chard seed crops (du Toit 2007). Approximately 95% of the table beet and Swiss chard seed crops grown in the United States are produced in this region. The winters are cold enough for vernalization to trigger bolting, but not so cold as to kill overwintering plants. This, coupled with the reasonably dry summers with low relative humidity, creates optimal conditions for production of beet and chard seed crops (du Toit 2007; Organic Seed Alliance 2016; Rackham 2002; Schreiber and Ritchie 1995). Other regions of the world where beet and chard seed production occurs include Chile, northern Europe, Newzealand, South Africa (Jacobsen 2009).Pseudomonas syringae pv. aptata causes bacterial leaf spot of all B. vulgaris subspecies (Jacobsen 2009).Over the past several decades, bacterial leaf spot has gained greater economic importance in table beet and Swiss chard seed production due to the rising demand for seed to plant the expanding acreage of baby leaf beet and chard crops in the United States (Crane 2023). This increase in baby leaf production is driven partly by increased public awareness of the nutritional benefits of vegetables and the convenience of pre-packaged salads (Lin et al. 2003).The objectives of this study were to: i) determine the prevalence and genetic diversity of P. syringae pv. aptata in table beet and Swiss chard seed crops in western Washington, and ii) identify the genetic basis of pathogenicity to beet and chard that differentiates strains of P. syringae pv. aptata from non-pathogenic strains of P. syringae associated with beet and chard plants, and seed, and differentiate strains pathogenic on beet and chard vs. beet only. The latter would facilitate development of molecular diagnostic tools to differentiate the pathogen from non-pathogenic strains, and enable rapid quantification of the pathogen in beet and chard seed lots.