Contributions of Reduced Susceptibility Alleles in Breeding Apple Cultivars with Durable Resistance to Fire Blight

Abstract

Breeding apple cultivars with durable genetic resistance is a potential long-term solution to fire blight, a devastating bacterial disease caused by Erwinia amylovora. However, phenotyping resistance/susceptibility to fire blight is challenging due to E. amylovora strain virulence, differential host × strain interactions, quantitative host resistance, environmental influences on disease, and impacts of tree vigor on susceptibility. Inheritance of resistance/susceptibility to fire blight is complex and phenotypic information alone is insufficient to guide breeding decisions targeting resistance. Several quantitative trait loci (QTLs) associated with resistance/susceptibility to fire blight have been detected throughout the apple genome. Most resistance alleles at fire blight QTLs have been identified in wild Malus germplasm with poor fruit quality, which limits their breeding utility. Several QTLs have been identified in populations derived from cultivars and reduced-susceptibility alleles have been characterized in multiple important breeding parents. Although resistance to fire blight is an attractive target for DNA-informed breeding, relatively few trait-predictive DNA tests for breeding relevant fire blight QTLs are available. Here we discuss (1) considerations and challenges associated with phenotyping resistance/susceptibility to fire blight; (2) sources of resistance that have been identified for use as parents; and (3) our perspective on short and long-term strategies to breed apple cultivars with durable resistance to fire blight with emphasis on the potential contributions of reduced susceptibility alleles to achieve this goal.

Identification of additional fire blight resistance candidate genes following MinION Oxford Nanopore sequencing and assembly of BAC clone spanning the Malus fusca resistance locus

“…genepool, although there is consensus amongst the research community that breeding fire blight resistant cultivars (Peil et al 2021a) is the best strategy to mitigate the devastating effects of the disease, caused by the bacterium Erwinia amylovora (Burrill; Winslow et al 1920). The devastation of fire blight is due largely to the pathogen strainspecificity of susceptibility/resistance found in the genus Malus (Norelli and Aldwinkle 1986;Peil et al 2011;Vogt et al 2013;Wöhner et al 2018;Emeriewen et al 2019), coupled with the fact that most apple cultivars are highly susceptible to the disease (Kostick et al 2021a). Therefore, the focus of breeding and breeding research has been to identify genomic regions in Malus that are associated with reduced susceptibility and resistance to fire blight, and combining these factors to achieve durable resistance (Peil et al 2021b).…”

“…As a consequence, several quantitative trait loci (QTL) and markers linked to resistance were identified in apple cultivars (Calenge et al 2005;Le Roux et al 2010;Desnoues et al 2018;van de Weg et al 2018;Kostick et al 2021b;Peil et al 2021a). It has been argued, and there is evidence that the loci of apple cultivars are associated with reduced susceptibility rather than complete resistance to fire blight (Kostick et al 2021b); nevertheless, they are important in achieving durable resistance (Kostick et al 2021a). In contrast, a few wild Malus genotypes possessing strong fire blight resistance have been reported with major QTLs detected in these genotypes (Peil et al 2007(Peil et al , 2019Durel et al 2009;Emeriewen et al 2014Emeriewen et al , 2017a.…”

Identification of Apple Varieties Resistant to Fire Blight (Erwinia amylovora) Using Molecular Markers

“…The breeding-relevant QTLs that have been previously characterized can be targeted to develop DNA tests for breeders to pyramid favorable alleles and/or combine superior fruit quality with fire blight resistance. The introgression and pyramiding of favorable alleles can be accelerated by fast-cycle breeding techniques [18]. The objective of this research was to analyze domestic and foreign apple varieties using SNP and SCAR markers to identify genetic sources of fire blight resistance.…”

“…Therefore, optimal methods for assessing genetically determined resistance to pathogens will always be in demand. Breeding apple tree varieties with genetic resistance offers a solution to fire blight [18].…”