Polyploidy affects the development of Venturia inaequalis in scab-resistant and -susceptible apple cultivars

Apple (Malus x domestica Borkh.) production is significantly affected by apple scab disease caused by the fungus Venturia inaequalis. Currently, management of the disease is largely based on fungicide applications and less on the inherent resistance in apple cultivars carrying resistance (Rvi) genes. Moreover, the durability of both approaches is questionable, since new virulent strains with resistance to fungicides and/or Rvi genes can develop. Hence, a combination of complementary management strategies is needed to tackle apple scab. To broaden the management options, a viable approach might be the breeding for polyploid cultivars and their implementation in commercial orchards. Apple germplasm is substantially characterized by polyploidy, which throughout the evolution of angiosperms often led to improved adaptation to adverse climatic conditions and abiotic stresses. However, the role of polyploidy in conferring resistance to biotic challenges is less clear. Here, we show that autopolyploidy in two apple cultivars, i.e. the susceptible 'Gala', and the resistant 'Makali' which carries the Rvi6 scab resistance gene, can reduce severity of apple scab symptoms upon inoculation with two different V. inaequalis isolates. Leaves of tetraploid plants show reduced sporulation symptoms in comparison with their diploid counterparts upon infection with both pathogenic strains, attributed to a reduced presence of V. inaequalis. These results suggest that polyploidy enhances resistance of apple cultivars against different V. inaequalis isolates. However, the degree of the enhanced resistance depends on a plant-pathogen-specific interaction. The knowledge provided here offers a relevant framework for improving our understanding of polyploidy-enhanced resistance to biotic stress in plants and may set the stage for implementing polyploidy breeding for further genetic improvement of apple.