Inheritance of resistance to common bacterial blight (Xanthomonas axonopodis phaseoli) disease of common bean
Osundwa, Cynthia Ogwogu
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Common Bacterial Blight (CBB), caused by Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans (designated as Xanthomonas axonopodis pv. phaseoli variant fuscans) is a major constraint to bean production in Uganda. The bacteria infect all aerial plant parts, including pods and seeds. Genetic resistance is the most ideal approach to the management of CBB. The key aim of this study was to identify genotypes with combined leaf and pod resistance to four CBB strains. A total of 20 genotypes with known state of resistance and two checks were re-evaluated in the screen house for CBB disease resistance in a split plot design replicated twice. Individual genotypes were inoculated with four CBB isolates; (MBL 020, XAP 1680, MBL 006 and MSK 005). To determine the mode of inheritance, six parents, whereby four resistant’ (VAX 6, VAX 4, VAX 3, NE 2-14-8) and two susceptible (Masindi Yellow and Bumwufu) were crossed in a complete diallel mating scheme with reciprocals. The F1 progenies were advanced to F2. The F2 were then evaluated for CBB disease resistance in a Randomized Complete Block Design together with the parents under screen house conditions. Data for both screening and inheritance study were collected on CBB disease severity and analysed using GenStat 18th Edition. A highly significant (P< 0.001) genotype effect on CBB severity on both leaf and pod was observed indicating high genetic diversity among the tested germplasm. CBB severity means ranged from 3.5 to 9.00 on leaves across strains whereas on pods it ranged from 3.63 to 8.3. The isolate effect was non-significant implying low genetic variability among the four isolates. Relatively moderate significant (P<0.001) Pearson correlation coefficients were detected between leaf and pod reactions with a regression of 0.41. The genotypes VAX 3, VAX 4, VAX 6 and NE-2-14-8 consistently showed intermediate resistance to the four pathogen isolates on both leaves and pods and therefore were selected for utilization to introgress CBB resistance to Ugandan preferred susceptible varieties. The reciprocal effects were non-significant in the F2 progenies suggesting that cytoplasmic factor was not important in the set of genotypes studied. General combining ability (GCA) was highly significant (P< 0.001) for both leaf and pod infection, whereas specific combining ability (SCA) was non-significant suggesting additive gene effects were predominant over non-additive in both leaf and pod CBB resistance. The resistant parent VAX 3 and VAX 4 had good aptitude of transmission for resistance to CBB in leaf and combined leaf and pod respectively. These parents are therefore promising sources of resistance that can effectively be used for transferring CBB resistance. Two crosses; Masindi Yellow × VAX 6 and VAX 4 × NE 2-14-8 presented a significant negative SCA effect (P< 0.05) for leaf and both leaf and pod respectively. This indicated the presence of non-additive gene effects for CBB resistance in these crosses. The estimated narrow sense of genetic determination was moderately high (0.58) for the resistance in leaf and high (0.87) in pod resistance suggesting early generation selection to be effective. Baker’s ratio estimates were relatively high for resistance in leaf (0.88) and pod (0.95) suggesting that hybrids’ performance can be predicted based on the parents’ general combining ability (GCA) effects. The inheritance of common bacterial blight was majorly controlled by recessive genes.