Mapping of a New Locus for Powdery Mildew Resistance in the Caucasus Apple <scp> <i>Malus orientalis</i> </scp>

ABSTRACT Powdery mildew is one of the most significant fungal diseases affecting apple, both the tree and its fruits, causing considerable economic losses to apple growers. Controlling this disease can require the application of multiple fungicides. A more sustainable approach is to grow resistant cultivars. In this study, we investigated the genetic basis of powdery mildew resistance in the wild apple accession Malus orientalis MOR17/1. To conduct genetic mapping, a crossing population was established between ‘Golden Delicious’ and MOR17/1. A total of 172 progeny were phenotyped under greenhouse and field conditions. The correlation between greenhouse and field data is low. Genotyping was performed employing a set of 17 SSR markers and the Axiom JKI50KMd SNP‐array. A high‐density linkage map was calculated for MOR17/1. The greenhouse data were used for mapping as single locus with JoinMap5.0 and together with the field data as quantitative trait loci (QTL) using MapQTL5.0. Mapping of the greenhouse data revealed a locus on LG02 for both phenotyping time points, as well as for the mean and maximum values. The highest LOD, 45.7, was observed for the second phenotyping. QTL mapping of field data from 3 years of observations confirmed the resistance locus on LG02 at a lower LOD for the second phenotyping in 2023 and for both phenotypings in 2025, as well as for the mean and maximum values. An additional QTL was detected at the bottom of the LG02 only in the first phenotyping in 2025. In the first phenotyping in 2024, a QTL was detected on LG11. A new locus conferring resistance to powdery mildew was mapped on LG02 of MOR17/1. These findings expand the pool of known powdery mildew resistance genes available to the breeders and offer new possibilities for resistance pyramids.