Genetics of leaf rust resistance in the spring wheats ‘Ivan’ and ‘Knudson’ spring wheat

Leaf rust caused by Puccinia triticina is a common and widespread disease of wheat in North America and worldwide. Durable genetic resistance to leaf rust in wheat has been difficult to achieve, since the virulence of the leaf rust pathogen to specific leaf rust resistance genes in wheat is highly variable. ‘Ivan’ and ‘Knudson’, hard red spring wheat cultivars recently released by AgriPro®, are highly resistant to leaf rust. The objective of this study was to determine the identity of leaf rust resistance genes present in both cultivars. ‘Ivan’ and ‘Knudson’ were crossed with ‘Thatcher’, a cultivar susceptible to leaf rust; the F₁ plants were backcrossed to ‘Thatcher’; and the BCF₁ plants (approximately 80 from each cross) were selfed to develop BCF₂ families. The BCF₂ families were tested as seedlings with different P. triticina isolates to determine the number and identity of leaf rust resistance genes that segregated in the BCF₂ families. Both the BCF₃ lines from selected resistant BCF₂ plants and near-isogenic lines of ‘Thatcher’ were tested with different P. triticina isolates to confirm the identity of the resistance genes. Selected BCF₂ families were also tested as adult plants in the greenhouse and field to identify genes for adult plant leaf rust resistance. ‘Ivan’ was determined to have genes Lr16 and Lr24 and ‘Knudson’ to have Lr3, Lr10, Lr13, Lr16, Lr23, and Lr34. ‘Ivan’ has been highly resistant because of the rarity of leaf rust isolates with virulence to Lr16 and Lr24, while the combination of Lr16, Lr23, and Lr34 accounts for the resistance in ‘Knudson’.