Response in Different Plant Parts to Xanthomonas campestris pv. phaseoli Strains for Breeding Common Bean with High Levels of Common Blight Resistance.

Common blight caused by Xanthomonas campestris pv. phaseoli is a severe disease of common bean (Phaseolus vulgaris L.) in the tropics and subtropics. Use of resistant cultivars is crucial for its control. Low levels of resistance occur in the primary and secondary gene pool. But, high levels of resistance are found in the tertiary gene pool. Resistance QTL respond differently to contrasting bacterial strains in different plant parts. The objectives were to: (1) determine the response in leaves and pods to two bacterial strains for 27 genotypes with different combinations of resistance QTL, (2) assay for the presence or absence of SAP6, BC420, and SU91 resistance QTL, and (3) identify genotypes with higher levels resistance. Twenty eight common beans including the susceptible check pinto ‘Othello’ were screened in the greenhouse. Less aggressive ARX08AC and aggressive Xcp25 bacterial strains were inoculated in primary and trifoliolate leaves and pods at 1.7 x 10⁸ CFU/ml density. Othello was susceptible (scores of 9.0) in leaves to both bacterial strains. ICB-3 and SE153, with SAP6 QTL, were resistant to ARX08AC (mean scores < 3.5) and susceptible to Xcp25 (≥ 6.5) in the trifoliolate leaf. USPT-CBB-5 and VAX 6, with SAP6 and SU91 QTL, were resistant in both leaf types to ARX08AC. But, USPT-CBB-5 was susceptible to Xcp25 in the trifoliolate leaf. Andean AM101 had the lower mean scores (≤ 2.4) in both leaf types and all plants were resistant in the trifoliate leaf to both strains. But, AM101, RCS63, and Wilkinson 2, with BC420 and SU91 QTL, were susceptible, and only VAX 6 had 11% of resistant plants (range 3-6) to both bacterial strains in pods. Thus, it would be important to identify new genes/QTL across Phaseolus species that confer high levels of broad-based resistance in all plant organs, especially pods.