An image-analysis based leaf disc assay for the rapid evaluation of genetic resistance to fire blight in apples

Fire blight, caused by Erwinia amylovora , is a destructive bacterial disease that severely hampers apple production. To conduct Quantitative Trait Locus (QTL) studies for breeding resistant apple cultivars, phenotyping of large genetic mapping populations of apples for fire blight resistance is essential. This, however, necessitates precise, quantitative data spanning multiple years, locations, and pathogen strains. It can be time-consuming and resource-intensive to keep QTL mapping populations for apples in the field and greenhouse. This creates a bottleneck for identifying novel QTL for fire blight resistance or developing resistant cultivars. To address this challenge, we present an image-based method for rapid and accurate phenotyping fire blight resistance using apple leaf discs. This leaf disc assay demonstrates significant ( p < 0.05) percent disease area (PDA) differences in fire blight inoculations among eight apple genotypes with well-known resistance levels. Furthermore, the image-based leaf disc assay consistently shows a 40–70% difference in PDA between resistant and susceptible checks. We also report high within and across trial broad sense heritability values ranging from 0.86 – 0.97. We demonstrate the use of K-means clustering and best linear unbiased estimators to combine multiple trials. This assay offers an efficient alternative to traditional fire blight screening methods, potentially improving our understanding of the host response, and accelerating the development of resistant apple cultivars.