Discovery of the Wild Sunflower-Derived Novel Downy Mildew Resistance Gene Pl19 in Sunflower (Helianthus annuus L.).

Downy mildew caused by Plasmopara halstedii is one of the most destructive diseases of sunflower production in the world. Historically, host genetic resistance has provided an economically and environmentally friendly method for controlling downy mildew. However, the main drawback of using R genes to control resistance is that their effects are often not durable because of the rapidly evolving P. halstedii pathogen. This necessitates the continued search for new sources of resistance and designing new strategies for more durable resistance. Wild sunflower species are an important source of downy mildew resistance genes. A wild Helianthus annuus accession PI 435414 was found to have resistance to downy mildew. Inoculation of 140 BC₁F_2:3 families derived from the cross of CMS CONFSCLB1 and PI 435414 with the virulent race 734 of P. halstedii revealed that a single dominant gene controls downy mildew resistance in the population. Bulked segregant analysis conducted in the BC₁F₂ population with over 860 simple sequence repeat (SSR) markers indicated that the resistance derived from the wild H. annuus was associated with the SSR markers located on linkage group (LG) 4 of the sunflower genome. To map and tag this resistance locus designated Pl₁₉, the 140 BC₁F₂ individuals were used to construct a linkage map at the gene region. Two SSR markers, ORS963 and HT298, were showed to be linked to Pl₁₉ within a distance of 4.7 cM. After screening 27 additional single nucleotide polymorphism (SNP) markers previously mapped to this region, two flanking SNP markers, NSA_003564 and NSA_006089, surround the Pl₁₉ gene at 0.6 cM from each side. This new gene is highly effective against the most predominant and virulent races of P. halstedii currently identified in North America, providing a novel gene for use in sunflower breeding programs.