Response to Selection for Resistance to Common Bacterial Blight in Recurrent Backcrosses in Large-Seeded Andean Common Bean.

Common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli (Smith) Dye and X. campestris pv. phaseoli var. fuscans causes severe losses to yield and quality in common bean (Phaseolus vulgaris L.). Resistance to CBB is quantitatively inherited, with low to moderately high heritability. However, previous studies suggested that CBB resistance could be retained even after five recurrent backcrosses. The objective of this research was to assess the response to one and two recurrent backcrosses for the introgression of CBB resistance into susceptible dark red kidney (DRK) bean. A double-cross [Wilkinson 2 (R=Resistant) / DRK 2 (S=Susceptible) // DRK 1 (S) / VAX 3 (R)] was made to introgress CBB resistance from the Middle American gene pool into DRK bean. The double-cross F1 of 707 plants was assessed in the greenhouse using direct pathogen inoculation scored on a 1-9 scale (1<4 = resistant, 4<7 = intermediate, 7-9 = susceptible). Selected resistant plants were backcrossed with DRK 1 (2188 BC1F1 seeds). The BC1F1 was screened for CBB resistance and selected resistant plants backcrossed with DRK 1 (1080 BC2F1 seeds). The BC1F2 and BC2F1 and their progenies were assessed for CBB resistance in the field and greenhouse. The fifteen breeding lines with the highest levels of CBB resistance from each backcross were selected. The four parents and 15 BC1 and 15 BC2 lines were evaluated for CBB reaction in the greenhouse. The mean disease severity index (DSI) for the 15 breeding lines was 4.3 for the BC1 and 5.7 for the BC2 compared with 7.5 for DRK 1 and 7.2 for DRK 2. After a single backcross, the majority of breeding lines remained resistant to CBB. However, after the second backcross, only one breeding line was rated resistant to CBB. The implications of these results for breeding will be discussed further.