Successful Introduction of Nonhost Resistance to Apple Scab Caused by Venturia inaequalis from Pyrus into Malus × domestica through Intergeneric Hybridization

Apple scab, caused by Venturia inaequalis, is a major disease in Malus × domestica. Although resistance genes such as Rvi6 have been introduced through breeding, the emergence of virulent pathogen races has raised concerns over the durability of single-gene resistance. Nonhost resistance (NHR), a broad-spectrum and durable form of immunity found in nonhost species, offers a promising alternative. This study assessed resistance to V. inaequalis in three Pyrus species and three intergeneric hybrids with M. × domestica. Following inoculation, macroscopic and microscopic evaluations revealed that susceptible Malus cultivars showed extensive lesion development and hyphal elongation. In contrast, no typical scab symptoms were observed in Pyrus, and hyphal growth was inhibited after initial conidial germination, indicating the presence of NHR in Pyrus species, including both European and Asian pears, against V. inaequalis. The intergeneric hybrids exhibited resistance phenotypes consistent with Pyrus-derived NHR. Our findings demonstrated that NHR can be inherited by Malus–Pyrus hybrids, suggesting that Pyrus could serve as a genetic source of novel, durable resistance for scab-resistant apple breeding.