Phenotypic Evaluation and Genome-Wide Association Mapping for Bacterial Leaf Streak Resistance in a Worldwide...

Bacterial leaf streak (BLS), caused by the bacterial pathogen Xanthomonas translucens pv. undulosa (Xtu), is an important disease affecting wheat production worldwide. Recently, the Northern Great Plains of the United States-a key region for durum wheat and hard red spring wheat-has reported an increased prevalence and severity of BLS. Growing resistant wheat varieties remains the most effective and environmentally friendly strategy to mitigate yield losses caused by this disease. The extensive virulence and genetic diversity of Xtu strains, coupled with the quantitative genetic architecture of wheat-pathogen interactions, highlight the critical need to identify diverse sources of resistance. Cultivated emmer wheat offers a broad genetic diversity that could be leveraged for resistance breeding. In this study, we evaluated BLS disease severity in 508 cultivated emmer wheat lines at the seedling stage under controlled environmental conditions. Fourteen emmer wheat lines exhibited high levels of resistance to BLS. One quantitative trait locus (QTL) on chromosome 1A was consistently identified across multiple models from genome-wide association mapping, accounting for 6.1% of the total phenotypic variation. The favorable allele of the most significant marker, S1A_305713424, was observed to be absent in both durum wheat landraces and breeding lines, suggesting that this favorable allele may have been lost during domestication from cultivated emmer wheat. These findings may represent a valuable germplasm resource for breeding BLS-resistant durum and bread wheat varieties. Further investigation is needed to validate the QTL effect in durum wheat, identify the candidate gene, and elucidate the underlying resistance mechanism associated with the identified QTL.