Snap beans or green beans have been selected for high pod quality with reduced fiber and are consumed as green pods harvested for the fresh market or processing. Slender or ”small-sieve” snap beans or green beans, also known as “French beans” are an important source of income for smallholder growers in East Africa who produce the crop for export markets typically in Western Europe [CBI, 2006; Centro Internacional de Agricultura Tropical (CIAT), 2006; Muchui et al., 2008; Okello and Roy, 2007], but they are also increasingly becoming important for domestic markets where they provide an important source of nutrition (CIAT, 2006; Kinyuru et al., 2011).
Production is concentrated at elevations above 1500 m at the east side of Lake Victoria, where they are grown during the rainy seasons and are harvested by hand up to six times in each season. The primary constraints to production are caused by high night temperatures, which restrict growing areas to higher altitudes, and airborne diseases in particular, common bean rust caused by the fungus Uromyces appendiculatus, which can severely limit the yield and quality of the snap bean crops (CIAT, 2006; Wasonga et al., 2010; Wortmann et al., 1998). Rust is a destructive disease of common beans in subtropical and tropical regions and is particularly severe in eastern and southern Africa (Kimani et al., 2002; Liebenberg et al., 2006; Wortmann et al., 1998). Heat stress and rust occur within the same production regions in East Africa (Wasonga et al., 2010) reducing both the quality and yields of snap bean crops generated.
The majority of snap bean cultivars grown in East Africa are highly susceptible to rust (Kimani et al., 2002; Wasonga et al., 2010). Use of genetic resistance in the management of bean rust has been difficult to achieve as a result of the extensive and shifting nature of virulence diversity of the pathogen (Araya et al., 2004; Liebenberg, 2003; Markell et al., 2009; Wright et al., 2008) coupled with lack of a single resistance gene that could confer resistance against all the races of the pathogen (Pastor-Corrales, 2006; Stavely and Pastor-Corrales, 1989). The deployment of Andean Ur-4 in combination with Middle American Ur-11 rust-resistance genes provided effective resistance to rust at different test sites in East Africa (Wasonga et al., 2010)
Higher than optimal temperatures (heat stress) adversely affect crop growth and productivity (Challinor et al., 2007; Wahid et al., 2007). Snap bean production in East Africa is currently limited to cool highland areas above 1500 m, because higher night temperatures experienced at lower altitudes reduce yield and green pod quality. Furthermore, land areas that are suitable for snap bean production in the East Africa region are expected to reduce in size given climate change-associated temperature increases currently being experienced across sub-Saharan Africa (Hulme et al., 2001; King’uyu et al., 2000). These temperature increases in sub-Saharan Africa are expected to exceed projected global mean increase of 2.5 °C by 0.7 to 1.1 °C by the end of the 21st century (Christensen et al., 2007; Cline, 2007). There is therefore need to adopt crop-specific production approaches that would enable increased or sustained productivity under elevated temperature conditions.
Genetic improvement of snap bean for tolerance to high-temperature stress is a promising option for increasing yield and quality in heat-stressed environments (Porch and Jahn, 2001; Rainey and Griffiths, 2005). Wasonga et al. (2010) developed and tested in East Africa some heat-tolerant snap bean breeding lines that demonstrated potential for improving production in the region as well as other tropical and subtropical environments where high temperatures presently limit production and changing climatic conditions are likely to become more challenging.
Among the quality attributes used to categorize snap beans for East Africa production are a small pod diameter, commonly referred to as small-sieve, in a long, slender, dark green pod. Size is determined by the maximum width of the pod measured at right angles to the seam and is classified as follows: 1) extra fine: width of the pod not exceeding 6 mm (sieve size 1); 2) fine: width of the pod not exceeding 9 mm (sieve sizes 2 and 3); and 3) medium: width of the pod not exceeding 12 mm. In East Africa, small-sieve snap bean cultivars producing extrafine and fine pods are the grades that growers desire for the markets served (Muchui et al., 2008). The small-sieve snap bean cultivars currently grown in East Africa lack the optimal combinations of traits for more efficient production that would be provided by incorporation of rust resistance and tolerance to high-temperature stress.
The overall goal was to improve snap bean for rust resistance and heat tolerance to increase production in environments experiencing these challenges need to ensure that the optimal traits are combined in desired market types that are high-yielding with good pod quality. The specific objectives of this study were to: 1) combine heat tolerance and optimal rust-resistance gene combinations in small-sieve snap bean genotypes; and 2) evaluate the snap bean lines in five locations in East Africa and one site in Puerto Rico, which differ in climatic conditions and virulence diversity for the bean rust pathogen.
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