2018 ASHS Annual Conference

Spinach (Spinacia oleracea) is an important cool-season leafy vegetable crop. A significant increase in spinach consumption in last two decades in the U.S. is attributed to an increased consumer health-consciousness. Downy mildew (DM) caused by an obligate oomycete Peronospora effusa, is an economically important disease in spinach. Many new races (> 10) have been identified in last two decades and such a rapid increase in the number of races threatens the sustainable production of spinach. DM can be managed using a number of strategies, but disease resistance is the most practical and economical management practice. Disease resistance is particularly critical for organic spinach production, which makes up approximately 50% of the market. The development of durable resistance of the downy mildew pathogen is critical. A total of 400 spinach genotypes, collected from a wide geographical region and maintained at NCRPIS, were evaluated at the USDA research station, Salinas, CA in Oct. 2017. The purpose of the field evaluation was identify field resistance to the downy mildew pathogen under natural disease pressure under field conditions. Field screening for downy mildew resistance can potentially identify partial resistance governed by QTL. The selection for QTL resistance in spinach to the downy mildew pathogen can improve durability of resistance. The spinach accessions evaluated were originated from 37 countries and belong to five different species but most are S. oleracea. Disease severity was rated on a scale of 0-100% based on a percentage of leaf area showing signs or symptoms of downy mildew. A wide variation in downy mildew disease severity was observed among the evaluated spinach genotypes. A whole-genome resequencing of all evaluated spinach genotypes is underway, and the SNPs marker identified from the population-resequencing approach will be used to conduct genome-wide association analysis. Identification of markers associated with minor and major resistance alleles can be used for gene pyramiding to improve the durability of resistance of spinach cultivars to the spectrum of races of the downy mildew pathogen.