Resistance Screening and Association Mapping for Resistance to the Downy Mildew Pathogen of Spinach

Spinach is an important cool leafy vegetable cultivated around the world, with large scale production in California and Arizona in the U.S. Spinach is a highly nutritious vegetable beneficial in the human diet. Spinach is affected by a number of biotic stressors. Downy mildew, caused by the oomycete pathogen Peronospora effusa, is a major threat to spinach as it affects the leaf quality and impacts the economic value of spinach. Several efforts have led to the development of resistant genotypes/cultivars to this pathogen. However, few studies have examined the genetics of resistance to the downy mildew pathogen in detail. This study aimed at: (1) evaluating a hybrid and a biparental (F2) population for incomplete dominant resistance and complete dominant resistance respectively, at the RPF3 locus to Pfs race 5, (2) evaluating 39 contemporary commercial cultivars for resistance to race 19 (isolate UA2020-01E), and (3) evaluating and conducting an association analysis for resistance to the downy mildew pathogen, Pfs race 5 in a diverse spinach panel comprising of 251 genotypes. The results from the studies indicated that resistance conferred against Pfs race 5 when resistance alleles are present in heterozygous condition is complete and segregates as a single gene. Thus, resistance is conferred by a single gene, or few genes tightly linked at the RPF3 locus. Resistance to race 19 (isolate UA2020-01E) is conferred by resistance gene at the RPF3 locus, but some genotypes lacking resistance at the RPF3 locus were resistant to race 19 indicating resistance is conferred at a locus that has not been identified or named. Genotypes in the association panel highly resistant, intermediate in resistance, or highly susceptible to Pfs 5 were indentified and several SNP markers (including chr3_2019852, chr3_484345 and chr3_486179 found on chromosome 3) were identified and were highly associated with resistance to Pfs 5. Further (ongoing) analysis such as haplotype and candidate gene analysis will provide more information on the genetics of resistance to Pfs race 5 as revealed by this association analysis. The potential characterization of resistant alleles, identification of genes providing resistance, and further dissecting the mechanism of genetic resistance will ultimately aid the development of durable resistant cultivars