Resistance to Melampsora larici-epitea leaf rust in Salix: analyses of quantitative trait loci.

Quantitative resistance of Salix to Melampsora larici-epitea leaf rust was studied in 2 Salix mapping populations. One population was a backcross between a S. schwerinii x S. viminalis hybrid and S. viminalis, and the other was an F2 population between S. viminalis and S. dasyclados. A leaf disc bioassay was used to study the components of quantitative resistance (latent period, uredinia number, and uredinia size) to 3 isolates of the leaf rust. The analysis of quantitative trait loci (QTLs) revealed 9 genomic regions in the backcross population and 7 genomic regions in the F2 population that were important for rust resistance, with QTLs explaining 8-26% of the phenotypic variation. An important genomic region was identified for the backcross population in linkage group 2, where QTLs were identified for all resistance components for 2 of the rust isolates. Four of the QTLs had overlapping mapping intervals, demonstrating a common genetic background for latent period, uredinia diameter, and uredinia number. QTLs specific to some rust isolates and to some resistance components were also found, indicating a combination of common and specific mechanisms involved in the various resistance components. Breeding implications in relation to these findings are discussed.