2019 ASHS Annual Conference

Spinach (Spinacia oleracea) is an important cool-season leafy vegetable crop. A significant increase in spinach consumption in the 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 [=P. farinosa f. sp. spinaciae (Pfs)], is the most important disease affecting fresh-market spinach production in California and Arizona. Seventeen different races of Pfs have been reported in spinach and many of these races (> 10) were identified in the last two decades. Utilization of host genetic resistance is the most practical and economical disease management practice; it is particularly relevant in spinach as approximately 50% of the market is organic based. The rapid emergence of many new Pfs races, along with a regular breakdown of the resistance genes deployed in the major cultivars, indicates an urgent need to screen large germplasm set to identify novel qualitative and quantitative resistance alleles. More than 400 spinach genotypes, collected from 37 countries and maintained at NCRPIS, were evaluated at the USDA research station in Salinas, CA and the Yuma agricultural center at the University of Arizona from 2017-2019. Disease severity was rated on a scale of 0-100% based on a percentage of leaf area exhibiting downy mildew-infected tissue. A wide variation in downy mildew disease severity was observed among the evaluated spinach genotypes; GBS-generated SNPs were used to conduct an association analysis and significant SNPs were identified. Whole-genome resequencing of all evaluated spinach genotypes is underway. SNP markers identified from the population-resequencing approach will be used to conduct genome-wide association analysis. The purpose of the field evaluation was to identify resistance to the downy mildew pathogen under natural conditions. Field screening for downy mildew resistance can potentially identify partial resistance governed by QTL. Mapping of a cultivar segregating for Pfs 16 resistance and genome resequencing of bulked resistant and susceptible segregants from multiple populations segregating for Pfs 13 are being pursued. The selection for major and minor resistance in spinach to the downy mildew pathogen can improve the durability of resistance. Identification of markers associated with minor and major resistance alleles can be used in gene pyramiding to improve the durability of resistance of spinach cultivars against the various races of the downy mildew pathogen.