Screening of the USDA Core Collection of Common Bean for Resistance to Halo Blight.

Halo blight, caused by Pseudomonas syringae pv. phaseolicola (Psp), is a seed-borne bacterial disease of common bean (Phaseolus vulgaris L.), particularly in regions with a moderately cool and wet climate. A subset of 62 accessions from a total of 283 accessions of P. vulgaris from the USDA-NPGS core collection showed resistance to race 6 of Psp on the primary leaves. Further evaluation of these 62 accessions with race 6 of Psp was performed on trifoliate and pod infections under greenhouse conditions. Disease severity index on trifoliates and pods was scored based on a disease rating scale of 1 – 9 [1-3 = resistant (R), 4-6 = intermediate (I), 7-9 = susceptible (S)]. Of the 62 accessions evaluated, 15 accessions (19%) were classified as resistant to race 6 of Psp on trifoliates with a mean score of 1.9, while 29 accessions (42%) showed highest levels of resistance on pods with a mean score of 2.4 in comparison of USHBR6, a resistant pinto check with a mean score of 1.0 and 1.5 on trifoliate and pod infection, respectively. The results of correlation analysis revealed a significant but weak correlation (r = 16%) between the trifoliate and pod reactions, suggesting that leaf resistance does not always translate into pod resistance and therefore, independent mechanisms of resistance may be responsible. Of these resistant accessions, PI 313490, a black bean from Mexico, showed the highest levels of resistance with a mean score of 0.8, and 2.7 for trifoliate and pod infections, respectively. Additional evaluation of these selected accessions will be conducted on both trifoliate and pod infections under field conditions as well as the identification of genomic regions linked to higher levels of resistance to race 6 of Psp by using a Genome-Wide Association Mapping approach.