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Article|01 Feb 2021|OPEN
Fire blight QTL analysis in a multi-family apple population identifies a reduced-susceptibility allele in ‘Honeycrisp’
Sarah A. Kostick1, Soon Li Teh1, John L. Norelli2, Stijn Vanderzande3, Cameron Peace3 & Kate M. Evans1,
1Tree Fruit Research and Extension Center, Department of Horticulture, Washington State University, Wenatchee, WA 98801, USA
2United States Department of Agriculture, Agricultural Research Service, Appalachian Fruit Research Laboratory, Kearneysville, WV 25430, USA
3Department of Horticulture, Washington State University, Pullman, WA 99164, USA

Horticulture Research 8,
Article number: 28 (2021)
doi: 10.1038/hortres.2021.28
Views: 228

Received: 28 Sep 2020
Revised: 14 Dec 2020
Accepted: 17 Dec 2020
Published online: 01 Feb 2021


Breeding apple cultivars with resistance offers a potential solution to fire blight, a damaging bacterial disease caused by Erwinia amylovora. Most resistance alleles at quantitative trait loci (QTLs) were previously characterized in diverse Malus germplasm with poor fruit quality, which reduces breeding utility. This study utilized a pedigree-based QTL analysis approach to elucidate the genetic basis of resistance/susceptibility to fire blight from multiple genetic sources in germplasm relevant to U.S. apple breeding programs. Twenty-seven important breeding parents (IBPs) were represented by 314 offspring from 32 full-sib families, with ‘Honeycrisp’ being the most highly represented IBP. Analyzing resistance/susceptibility data from a two-year replicated field inoculation study and previously curated genome-wide single nucleotide polymorphism data, QTLs were consistently mapped on chromosomes (Chrs.) 6, 7, and 15. These QTLs together explained ~28% of phenotypic variation. The Chr. 6 and Chr. 15 QTLs colocalized with previously reported QTLs, while the Chr. 7 QTL is possibly novel. ‘Honeycrisp’ inherited a rare reduced-susceptibility allele at the Chr. 6 QTL from its grandparent ‘Frostbite’. The highly resistant IBP ‘Enterprise’ had at least one putative reduced-susceptibility allele at all three QTLs. In general, lower susceptibility was observed for individuals with higher numbers of reduced-susceptibility alleles across QTLs. This study highlighted QTL mapping and allele characterization of resistance/susceptibility to fire blight in complex pedigree-connected apple breeding germplasm. Knowledge gained will enable more informed parental selection and development of trait-predictive DNA tests for pyramiding favorable alleles and selection of superior apple cultivars with resistance to fire blight.