Quantitative variation and heritability estimates of fire blight resistance in a pedigree-connected apple germplasm set

Abstract

Breeding resistant apple cultivars is a long-term solution to fire blight, a devastating bacterial disease caused by Erwinia amylovora. However, most resistance sources have been characterized in wild genetic backgrounds with poor fruit quality. Additionally, strong influences of environment and tree vigor on susceptibility, quantitative resistance, and the disease’s erratic nature make phenotyping challenging. This study examined levels of resistance/susceptibility to fire blight among and within 32 full-sib families (n = 314 seedlings) in a pedigree-connected apple germplasm set. In 2016 and 2017, multiple actively-growing shoots per tree (approximately 3 trees per seedling) were challenged with E. amylovora. Responses were quantified as proportions of current season’s shoot lengths that were blighted (SLB). Within a year, seedling responses were estimated using mean adjusted SLB best linear unbiased predictions (adjusted SLB BLUPs). Responses ranged from highly resistant to highly susceptible (0.04–0.97 adjusted SLB BLUPs across years) and were relatively consistent between years (Spearman’s R2 = 0.55). K-means clustering was used to classify seedlings into resistance/susceptibility groups based on incidence, adjusted SLB BLUPs, maximum SLB, and maximum age of wood infected. Most families, including crosses between susceptible parents, demonstrated quantitative variation for resistance/susceptibility. Families derived from ‘Enterprise’ demonstrated low relative susceptibility. Depending on the year and model, average narrow-sense heritability estimates ranged from 0.22–0.49. Phenotypic information (1) increases understanding of variation and inheritance of resistance/susceptibility to fire blight in germplasm relevant to US apple breeding programs and (2) could be used in pedigree-informed QTL analyses to detect loci associated with resistance/susceptibility.