The presence of leaf rust resistance genes Lr2a, Lr14a, Lr16, Lr21, and Lr34 in Canada Western Red Spring wheat cultivars 1939–2024

Genetic resistance has played a significant role in reducing losses to wheat leaf rust, caused by Puccinia triticina Erikss. in Canadian wheat. Highly predictive molecular markers have been developed for some of the key resistance genes used in Canadian wheat. All the cultivars grown on a significant area for the largest wheat class in Canada, Canada Western Red Spring (CWRS), were genotyped using markers with high predictive value for genes Lr2a, Lr14a, Lr16, Lr21, and Lr34. From the 1930s to 1954, when Thatcher predominated, of the genes tested, only Lr14a was present in less than 15% of the CWRS seeded area. Selkirk with Lr14a and Lr16 was popular from 1955 into the 1960s, leading to an increase in the area seeded for both these genes. The frequency of Lr16 subsequently declined but then increased again in the 1980s, along with the multi-pest resistance gene Lr34. Lr21 and Lr2a were introduced in the 1990s and early 2000s, respectively, though the frequency of these genes in the CWRS crop has increased recently to very high proportions. Recently, the proportions of Lr2a, Lr21 and Lr34 have all increased to over 50% of the seeded CWRS area, while those for Lr16 and, particularly, Lr14a, declined. This is often due to leaf rust-susceptible cultivars being replaced with resistant cultivars. This analysis demonstrates that parental selection to include resistance genes and selection for resistance, result in cultivars with more complex combinations of resistance genes, providing better genetic protection for the CWRS wheat crop over time.