Identification of the genetic architecture of stem rust resistance in perennial ryegrass

Perennial ryegrass (Lolium perenne) is a major species in temperate grassland systems, important for its use in forage and turf production. However, foliar diseases, such as stem rust (Puccinia graminis f. sp. lolii), severely reduce seed yields in Denmark. The expected intensification of disease pressure due to climate change, coupled with restrictions on pesticide use in Europe, highlights the urgent need for sustainable control strategies. Developing durable resistant varieties offers a sustainable strategy for stem rust control. This study aims to dissect the genetic architecture underlying resistance and identify resistant genotypes to stem rust. A collection of 390 Lolium genotypes from diverse backgrounds was phenotyped against a Danish stem rust isolate, based on assessments of pustule type and disease severity. A range of responses, from complete resistance to high susceptibility, was observed. A genome-wide association study (GWAS) revealed multiple loci associated with the traits, and candidate genes were identified. To complement these findings and detect rare alleles potentially involved in resistance, two biparental mapping populations were developed from parents exhibiting contrasting resistance phenotypes against three European isolates, including the Danish isolate previously used, and one American isolate. Mapping analyses are in progress to refine the identification of resistance loci. Altogether, this work provides new insights into the genetic determinism of stem rust resistance and lays a foundation for the future characterization of the molecular pathways involved in grass species. In parallel, the identified resistance loci will support pre-breeding programs aiming to develop durable resistant varieties of Lolium perenne.