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Although fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases of apple (Malus x domestica) worldwide, no major, qualitative gene for resistance to this disease has been identified to date in apple. We conducted a quantitative trait locus (QTL) analysis in two F(1) progenies derived from crosses between the cultivars Fiesta and either Discovery or Prima. Both progenies were inoculated in the greenhouse with the same strain of E. amylovora, and the length of necrosis was scored 7 days and 14 days after inoculation. Additive QTLs were identified using the MAPQTL: software, and digenic epistatic interactions, which are an indication of putative epistatic QTLs, were detected by two-way analyses of variance. A major QTL explaining 34.3--46.6% of the phenotypic variation was identified on linkage group (LG) 7 of Fiesta in both progenies at the same genetic position. Four minor QTLs were also identified on LGs 3, 12 and 13. In addition, several significant digenic interactions were identified in both progenies. These results confirm the complex polygenic nature of resistance to fire blight in the progenies studied and also reveal the existence of a major QTL on LG7 that is stable in two distinct genetic backgrounds. This QTL could be a valuable target in marker-assisted selection to obtain new, fire blight-resistant apple cultivars and forms a starting point for discovering the function of the genes underlying such QTLs involved in fire blight control.

We used a new method called nucleotide-binding site (NBS) profiling to identify and map resistance gene analogues (RGAs) in apple. This method simultaneously allows the amplification and the mapping of genetic markers anchored in the conserved NBS-encoding domain of plant disease resistance genes. Ninety-four individuals belonging to an F1 progeny derived from a cross between the apple cultivars 'Discovery' and 'TN10-8' were studied. Two degenerate primers designed from the highly conserved P-loop motif within the NBS domain were used together with adapter primers. Forty-three markers generated with NBS profiling could be mapped in this progeny. After sequencing, 23 markers were identified as RGAs, based on their homologies with known resistance genes or NBS/leucine-rich-repeat-like genes. Markers were mapped on 10 of the 17 linkage groups of the apple genetic map used. Most of these markers were organized in clusters. Twenty-five markers mapped close to major genes or quantitative trait loci for resistance to scab and mildew previously identified in different apple progenies. Several markers could become efficient tools for marker-assisted selection once converted into breeder-friendly markers. This study demonstrates the efficiency of the NBS-profiling method for generating RGA markers for resistance loci in apple.

Although fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases of apple ( Malus × domestica) worldwide, no major, qualitative gene for resistance to this disease has been identified to date in apple. We conducted a quantitative trait locus (QTL) analysis in two F1 progenies derived from crosses between the cultivars Fiesta and either Discovery or Prima. Both progenies were inoculated in the greenhouse with the same strain of E. amylovora, and the length of necrosis was scored 7 days and 14 days after inoculation. Additive QTLs were identified using the mapqtl software, and digenic epistatic interactions, which are an indication of putative epistatic QTLs, were detected by two-way analyses of variance. A major QTL explaining 34.3–46.6% of the phenotypic variation was identified on linkage group (LG) 7 of Fiesta in both progenies at the same genetic position. Four minor QTLs were also identified on LGs 3, 12 and 13. In addition, several significant digenic interactions were identified in both progenies. These results confirm the complex polygenic nature of resistance to fire blight in the progenies studied and also reveal the existence of a major QTL on LG7 that is stable in two distinct genetic backgrounds. This QTL could be a valuable target in marker-assisted selection to obtain new, fire blight-resistant apple cultivars and forms a starting point for discovering the function of the genes underlying such QTLs involved in fire blight control. [ABSTRACT FROM AUTHOR]