Genetic mapping of rust resistance loci in biomass willow

Rust diseases caused by Melampsora spp. represent a major threat to the productivity of short rotation coppice (SRC) willows grown for biomass, causing yield losses of up to 40%. The routine use of fungicide in SRC plantations is not a viable option because of economic and environmental considerations; thus, breeding for rust resistance is a major target for willow breeding programmes. To characterise the genetic basis of rust resistance in willow and provide targets for use in future marker-assisted selections, quantitative trait analyses were performed using a large full-sib mapping population (K8) which segregates for rust resistance and several other important agronomic traits. Rust resistance in field conditions was assessed in three consecutive years. For a more detailed genetic dissection, laboratory inoculation tests using isolates of two distinct and prevalent pathotypes (LET1 and LET5) were also performed. For field-based resistance, a major quantitative resistance locus, designated SRR1 (Salix Rust Resistance 1), was detected in addition to several quantitative trait loci (QTL) of more modest effect. Inoculation test data also supported an important role for SRR1. Specific interactions between particular rust isolates and different QTL were detected, and QTL that only influenced resistance in field conditions were identified. The QTL reported here represent an important basis for the future development of markers for use in willow breeding programmes. As the linkage map for the K8 population is anchored to the Populus trichocarpa genome sequence, a more efficient marker development for future fine-scale mapping and candidate gene identification is possible.