The 2012 ASHS Annual Conference

Powdery mildew (PM), caused by the fungus Podosphaera clandestina, is one of the most serious sweet cherry diseases in the Pacific Northwest.  Incorporation of natural resistance into elite sweet cherry cultivars seems to be an effective way to reduce reliance on fungicide and pesticide use and facilitate the transition to sustainable production systems with resultant increase in industry profitability. Currently, most commercial sweet cherry cultivars are susceptible to PM.  PMR-1, an open-pollinated seedling of unknown origin with below average fruit quality, has been identified as immune to the disease.  Other cultivars identified to date with putative resistance to PM are ‘Chelan’, ‘Hedelfingen’, ‘Venus’, and ‘Moreau’, of which only ‘Chelan’ has barely sufficient fruit quality for early season fresh marketing and is widely cultivated in Washington.  Many elite commercial cultivars have been crossed with these parents, but nothing is known about the gene(s) underlying the resistance(s). An understanding of the genetic mechanisms underlying resistance or immunity will facilitate development of new cultivars that combine superior fruit quality with PM resistance. In this study, a total of 480 pedigree-linked individuals representing the U.S. sweet cherry Crop Reference Set and the Pacific Northwest Sweet Cherry Breeding Program’s (PNWSCBP) Pedigree Set were screened for PM susceptibility or resistance using a 6-point scale, from 0 indicating ‘no symptoms’ to 5 indicating ‘leaves completely infected.’ An Infinium array including approximately 150 cherry single nucleotide polymorphism (SNP) markers with sufficient polymorphism was chosen from a total of 6000 SNPs developed by the RosBREED project (www.rosbreed.org) to provide genome-wide markers for determining marker-locus-trait associations. The FlexQTL™ software was used to integrate both the phenotypic and genotypic data to identify QTLs underlying PM resistance/susceptibility in sweet cherry. We identified two QTLs on linkage group (LG) 1. The first QTL mapped between 104.9cM and 111.9cM and was flanked by SNPs RS1.26215806 and RS1.27968875, respectively. The other QTL mapped between132.4cM and 135.6cM and was flanked by SNPS RS1.33106240 and RS1.33907538, respectively. The haplotypes for these QTLs are discussed in relation to breeding for PM resistance in sweet cherry.