Developing molecular markers for marker assisted selection of fire blight resistant apple seedlings

Fire blight is a serious disease of pome fruit. To identify molecular markers linked to fire blight resistance in a population derived from the cross 'Idared' × Malus × robusta 5, a genome scanning approach was used. Seventy-eight simple sequence repeat (SSR) markers dispersed on the linkage groups of apple were tested for polymorphism in the progeny. Sixty-one polymorphic SSRs were bulked to perform multiplex PCR with up to eight different primer pairs. After separation of PCRs on an Abimed 3100 Genetic Analyser, 59 SSRs produced a total of 64 loci. Only 55 loci could be mapped to 14 linkage groups. The alignment of microsatellites on the 14 linkage groups was, in general, in alignment with these markers on published linkage maps. For determining susceptibility of progenies to fire blight, seedlings were propagated clonally by grafting. Four weeks later artificial shoot inoculation using E. amylovora strain 222 susceptibility was determined as length of necrotic shoot in relation to total shoot length. The severity of necrosis of progenies ranged from 0 to 99% with a mean of 35%. A Kruskal-Wallis-analysis yielded strong evidence for three SSR markers to be correlated with resistance to fire blight. All three SSRs are located on one arm of linkage group 3 and all contributing alleles are from the wild species accession M. robusta 5. Two of these SSRs, CH03E03 and CH03G07, span an interval of 11 cM, which can explain up to 80% of the phenotypic variation in fire blight resistance. These SSRs have to be proven for their suitability in marker assisted selection.