QTL analysis of Malus baccata 'Jackii'-derived offspring reveals a polygenic inheritance pattern of apple blotch...

Apple blotch, caused by Diplocarpon coronariae, is an increasingly important fungal disease that leads to premature leaf fall and significant yield losses in apple orchards. Breeding resistant cultivars offers a sustainable strategy to reduce disease impact, as all commercial apple cultivars are susceptible to this pathogen. This study aimed to investigate the inheritance of apple blotch resistance derived from Malus baccata 'Jackii' through artificial inoculations conducted between 2023 and 2025 in Dresden-Pillnitz, followed by QTL analysis, which has not previously been reported for this disease. Simple interval mapping was performed using phenotypic and genotypic data from 122 individuals of an F₁ population ('Idared' × M. baccata 'Jackii'), together with analyses of M. baccata 'Jackii'-derived open-pollinated populations. Our results indicate that resistance to apple blotch is a complex, polygenic trait, with four important QTLs identified on linkage groups 1, 2, 12 and 13. These four QTLs showed LOD scores between 4.86 and 8.37 and explained between 17.3 and 32.4% of the phenotypic variance. However, disease severity was strongly affected by inoculum, phenotyping method and environmental factors. These findings have direct implications for apple breeding programmes aimed at developing apple blotch-resistant cultivars, and should be validated under field conditions.