Targeting Gene Combinations for Broad-spectrum Rust Resistance in Heat-tolerant Snap Beans Developed for Tropical Environments

Abstract

Common bean rust disease (caused by Uromyces appendiculatus) and high temperatures (heat stress) limit snap bean (Phaseolus vulgaris) production in many tropical and temperate regions. We have developed snap bean lines combining broad-spectrum rust resistance with heat tolerance for tropical agroecosystems. Eight breeding populations were developed by hybridizing BelJersey-RR-15 and BelFla-RR-1 (each possessing the Ur-4 and Ur-11 rust resistance genes) and the heat-tolerant snap bean breeding lines HT601, HT603, HT608, and HT611. F2–F4 generations of the populations were evaluated under greenhouse conditions and selected for heat tolerance while simultaneously selecting for the rust resistance genes Ur-4 and Ur-11. Three heat-tolerant F5 lines, which were homozygous for Ur-4 and Ur-11 genes, were selected together with a rust-resistant but heat-sensitive control. These and 12 cultivars adapted to different geographical regions were evaluated for their reaction to rust and yield at six contrasting field sites in eastern Africa and their response to high temperature verified in Puerto Rico. Rust incidence and severity was high at three of the trial sites in eastern Africa. Two of the 12 cultivars were resistant to rust at most of these sites, and three of the four breeding lines were resistant at all sites. The Ur-11 gene effectively conferred rust resistance at all sites. Yield in Puerto Rico was strongly correlated (R2 = 0.71, P < 0.001) with that of the hottest site in eastern Africa, highlighting the similarity in genotypic response to high temperatures at the two distinct sites. The newly developed rust-resistant and heat-tolerant breeding lines showed stable yield at the eastern Africa sites with contrasting mean temperatures compared with the cultivars presently grown in the region. Two of these lines, HT1 and HT2, were confirmed to be homozygous for Ur-4 and Ur-11 and with high heat tolerance under both greenhouse and field environments. This research validates the effectiveness of targeted rust resistance gene combinations for tropical environments and the effective selection of high temperature tolerance traits correlating across multiple environments. The breeding lines HT1 and HT2 developed in this research could be used to improve snap beans for the tropics and other environments with similar constraints.