Genetics, Genomics, and Breeding for Fire Blight Resistance in Pear

Abstract

Fire blight, caused by the bacterium Erwinia amylovora (Burrill) Winslow et al., is the most serious disease affecting the European pear, Pyrus communis L., in North America, Europe, and the Middle East. Control of fire blight is difficult, thus rendering the development of resistant cultivars and rootstocks a high priority. The inheritance pattern of resistance is quantitative, and genetic control is polygenic with additive effects, along with an estimated narrow-sense heritability, from various populations, of 0.40–0.50. There is some evidence for major gene inheritance for resistance. There have been five published studies on presence of genetic markers linked to quantitative trait loci (QTL). Microsatellite or simple sequence repeats (SSR) markers have been the most used marker type, but amplified fragment length polymorphisms (AFLPs) and single nucleotide polymorphisms (SNPs) have also been used. In the first study of the progeny ‘Passe Crassane’ × ‘Harrow Sweet’, four putative QTLs have been identified, all detected in ‘Harrow Sweet’. A QTL is located on linkage group (LG) HS2a, a second on HS2b, a third on HS4, and a fourth on HS9. In a follow-up study with additional markers that merged HS2a and HS2b, a single QTL is identified controlling disease incidence, severity, and the incidence severity (ISV) index. In addition, three putative QTLs have been identified for disease incidence, severity, and ISV on HS04. In a study of the progeny of ‘Doyenne du Comice’ × Pyrus ussuriensis No. 18, putative QTLs have been identified on LG 11 of the P. ussuriensis parent. Another QTL identified on LG 4 of ‘Doyenne du Comice’ has suggested that resistance genes could be present in susceptible parents, as observed in conventional segregation studies. A follow-up study has identified a QTL on LG 9 of the resistant parent, and additional QTLs on LG 11, as well as on three other linkage groups, have been also found. Furthermore, four additional QTLs have been identified in ‘Doyenne du Comice’. In an interspecific seedling population of ‘PremP003’ (P. × bretschneideri × P. communis) × ‘Moonglow’ (P. communis), a major QTL is mapped to LG 2 of ‘Moonglow’, which co-locates with a LG 2 QTL found in ‘Harrow Sweet’. Three minor QTL have been identified on LGs 9, 10, and 15 of ‘PremP003’. The history of pear breeding for fire blight resistance and notable cultivar releases will be also discussed.