Multi-environment Selection of Small Sieve Snap Beans Reduces Production Constraints in East Africa and Subtropical Regions

Abstract

Small-sieve snap beans (Phaseolus vulgaris L.) are an important source of income for smallholder farmers in East Africa. In this region as well as in other tropical and subtropical environments, common bean rust, caused by Uromyces appendiculatus (Pers.:Pers.), and heat stress reduce the yield and quality of snap beans. Small-sieve rust-resistant snap beans and that are heat-tolerant were developed using heat-tolerant snap bean breeding lines that had broad-spectrum rust resistance conditioned by the combination of the Andean Ur-4 and Mesoamerican Ur-11 genes. The Ur-4 and Ur-11 rust gene combination confers resistance to 90 races of the hypervariable pathogen from different parts of the world, including East Africa, that are maintained at Beltsville, MD. Four breeding lines each having the combination of the two rust genes were crossed in a 4 × 5 diallel mating design with five susceptible small-sieve cultivars to give 20 F1 hybrids. The hybrid combinations were advanced through the F2, F3, and F4 generations with selection for heat tolerance, rust resistance, and pod quality to develop lines combining these traits. Twenty F5 breeding lines that had the combination of these traits were selected and evaluated in East Africa at four field sites selected on the basis of differences in altitude, climate, and virulence diversity of the bean rust pathogen and in Puerto Rico at a field site characterized by high temperatures. There was a significant positive correlation between ranks of heat stress influenced yield components (seeds per pod and total yield) at the hot field site and the controlled high-temperature (32/27 °C) greenhouse. Four of the breeding lines developed, L5, L9, L13, and L17, combined heat tolerance and rust resistance in the desired plant type with high yield and good pod quality. These lines are the first known small-sieve snap beans with the combination of traits for heat tolerance and broad-spectrum rust resistance conferred by the Ur-4 and Ur-11 genes. These results demonstrate ability to combine heat tolerance and rust resistance as important traits for adaptation of specific market classes of common bean to tropical and subtropical environments through targeted selection of multiple traits in controlled environments.