The host-pathogen interaction and breeding and genetics of resistance for common bacterial blight of common bean (Phaseolus vulgaris L.) caused by Xanthomonas campestris pv. phaseoli and X. c. pv. phaseoli var. fuscans

Common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli Smith (Dye) (Xcp, synonym: X. axonopodis pv. phaseoli [Smith] Vauterin et al.) and X. c. pv. phaseoli var. fuscans (Burkholder) Starr & Burkholder (Xcpf, synonym: X. fuscans subsp. fuscans sp. nov.) is the most economically important bacterial disease of common bean (Phaseolus vulgaris L.). Host resistance is the best method of management for this seed-borne disease. This thesis research centered on (1) assessing pathogen diversity in a dark red kidney (DRK) bean growing region in the Midwestern USA (central Wisconsin) and the interaction of eight pathogen genotypes with known CBB resistant germplasm; (2) comparing direct disease resistance selection (DDS) and marker-assisted selection (MAS) for improving CBB resistance in common bean; and (3) dissecting the linkage between a CBB resistance-linked marker and flower and seed color in breeding populations of the large-seeded DRK bean (Andean gene pool).;In the host-pathogen diversity study, a new Xcp genotype was identified based on colony morphology, brown pigment production and rep-PCR fingerprint. A new Xcpf genotype was also identified based on rep-PCR fingerprinting, and this strain appeared to be a hybrid of the two previously reported genotypes. The new Xcp and Xcpf genotypes from Wisconsin, along with five previously reported genotypes, were individually inoculated onto 27 common bean genotypes, which possess previously identified sources of CBB resistance. Eight host genotypes were identified that were resistant to all pathogen genotypes, and all of these had pyramided resistance. This research revealed that common bean breeders have a diverse set of germplasm for CBB resistance breeding.;The second objective was to compare DDS with MAS for the development of CBB-resistant breeding lines, and to compare the cost-effectiveness of the two selection methods. Based on results of screening performed in a greenhouse, a greater number of CBB-resistant breeding lines were generated by DDS compared with MAS. However, under moderate disease pressure in the field, there were no significant differences in CBB resistance for breeding lines generated by the two selection techniques. Thus, both methods worked well for developing CBB-resistant breeding lines, but environment and disease pressure impacted selection of resistant breeding lines. Furthermore, under the parameters of this study, DDS was more cost-effective than MAS.;During the course of developing CBB-resistant breeding lines, an association between CBB resistance and flower and seed color was observed in segregating populations. Thus, the third objective was to investigate the relationship between flower and seed color, CBB resistance, a CBB resistance SCAR marker (BC420) and the SOD12490 marker associated with the V pigmentation locus. In an F2 population and in the F3 progeny test, no recombination was found between colored flowers (purple [P] and lilac [L]), a dark undesirable DRK seed coat color and presence of the BC420 marker. All plants in the F2 and F3 that lacked the BC420 marker had white/lilac-striped (wl) flower color and the desirable DRK seed color. Some plants with wl flowers were CBB-resistant but lacked the BC420 marker, indicating that unidentified QTL exist in the CBB donor germplasm (VAX 3, Wilkinson 2, or I9365-25). These QTL should be identified for use in CBB resistance breeding programs. Finally, these results also indicated that the SOD12490 marker was not tighly linked to CBB resistance as it was detected in intermediate and resistant plants.;In summary, the findings from this research extend our understanding of pathogen diversity, and reveal high levels of CBB resistance in a diversity of common bean germplasm. It was also demonstrated that MAS may not always be the best selection technique for CBB resistance breeding. A high density of markers associated with the majority of resistance QTL with small and large effects should be generated to enhance the utility of MAS. Finally, although the BC420 marker was a highly effective CBB resistance marker, it is closely linked with an undesirable seed coat color, making it difficult to utilize in a breeding program where certain seed colors are required. Together, this information will facilitate the improvement of CBB resistance in all market classes of common bean.