Pfeifer, M.; Peil, A.; Flachowsky, H.; Emeriewen, O. F.; Woehner, T. W.

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 disease resistance of Malus baccata 'Jackii'-derived offspring to D. coronariae through artificial inoculation and to identify loci associated with resistance. Simple interval mapping was performed using phenotypic and genotypic data from 122 individuals of an F1 population ('Idared' x 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. 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.