Characterization of resistance to fire blight (Erwinia amylovora) and blue mold (Penicillium expansum) in exotic and domesticated Malus (apple) germplasm

Genetic improvement of host resistance has been identified by the U.S. apple industry as a major way to mitigate production risks associated with economically important diseases such as fire blight and blue mold. Several fire blight resistance QTLs have been described in Malus, however most come from exotic germplasm of poor fruit quality. Two bi-parental populations derived from ‘Splendour’ apple, which has excellent flavor and is resistant to fire blight, were found to segregate for resistance to fire blight (1R:3S) when challenged with Erwinia amylovora. These populations are being genotyped to identify the genetic basis of this resistance. Another approach to identify marker trait associations for fire blight resistance within domesticated Malus cultivars is to utilize the pedigree-linked germplasm of the RosBREED project. The Malus Crop Reference and Washington State University (WSU) Breeder Pedigree Sets were propagated and a replicated research orchard will be established at WSU, Wenatchee, WA in spring 2015 and used to determine the resistance of this germplasm to fire blight. Although there is no known resistance to blue mold in M. ×domestica, M. sieversii PI613981 collected from the wild in Kazakhstan is resistant. Fruit collected from a ‘Royal Gala’ X PI613981 mapping population (GMAL4593) were inoculated with P. expansum and evaluated for decay over a three year period. A genetic framework map has been developed for the GMAL4593 population and was used to identify QTLs for blue mold resistance on linkage groups 4 and 10.