Lili Qi, Zahirul Talukder, Brent Hulke, Michael Foley
Abstract: Downy mildew, which is caused by fungus Plasmopara halstedii (Farl.) Berlese & de Toni, is one of the most important diseases that affect sunflower production globally. Two downy mildew resistance genes, PlArg and Pl8, were discovered in the late 1980s. Over two decades, PlArg is still effective against all known P. halstedii races identified to date, and Pl8 confers resistance to 96 % of the P. halstedii isolates recently collected in the USA. PlArg and Pl8 have been previously mapped to the linkage groups 1 and 13, respectively, of the sunflower genome using simple sequence repeat (SSR) markers. In this study, we developed high-density single nucleotide polymorphism (SNP) maps encompassing the Plarg and Pl8 genes and identified diagnostic SNP markers closely linked to these genes. The flanking SNP markers NSA_007595 and NSA_001835 delimited PlArg to an interval of 2.83 Mb and the SNP markers SFW01497 and SFW06597 delimited Pl8 to an interval of 2.85 Mb on the physical map of the sunflower genome. The specificity of the diagnostic markers was validated in a highly diverse panel of 548 sunflower lines. We also developed sunflower lines with homozygous, three gene pyramids carrying PlArg, Pl8, and the sunflower rust resistant gene R12. These were selected using the linked SNP markers from a segregating F2 population of RHA 340 (carrying Pl8)/ RHA 464 (carrying PlArg and R12). The pyramided sunflower lines will potentially provide durable resistance to downy mildew and rust.
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