Inheritance of resistance to new isolates of Xanthomonas campestris pv. malvacearum (Smith) dye, in Upland Cotton

Abstract

Resistant cultivars offer the most economical means of controlling bacterial blight of cotton. New isolates of the pathogen have evolved in Africa which are virulent on previously resistant cultivars. The nature and mode of inheritance of resistance to four new isolates of the bacterial blight pathogen, Xantomonas campestris pv. malvacearum, was investigated. Two parental sets of three cotton cultivars and their F1, F2 and backcross progenies, obtained from a half diallel mating scheme, were evaluated in the field and greenhouse for disease reaction to four African isolates and a mixture of USA races. True leaves in the field, and true leaves and cotyledons in the greenhouse, were graded on a scale of 1 to 10 for disease severity. A cultivar designated S295, recently developed in Chad, was reported as being resistant to the HV1 African isolate and was included in one parental set. Data were analyzed by generation means and Mendelian analyses. Resistance to the less virulent African isolates HV3, HV7, and Sudan, appears to be quantitative in nature. Dominance, when significant, was in the direction of resistance in all but two cross-isolate combinations. This indicated the influence of the gentic background on the direction of dominance. A significant portion of additive genetic variability accounted for the observed differences in generation means. Digenic non-allelic interactions were significant in most genetic models and indicated a duplicate type of interaction. Narrow sense heritability estimates ranged from 0.19 to 0.68 for resistance to the African isolates. Mendelian segregation for resistance to the HV1 isolate was observed when Stoneville 825 was crossed to S295, indicating a single major gene with complete dominance. However, the genetic background influenced this source of resistance. When Tamcot CAMD-E was crossed to S295, resistance to the HV1 isolate appeared to be quantitative. This major gene for resistance to X. c. pv. Malvacearum identified in S295, was designated "B12". Mendelian segregation was also observed for resistance to the USA race mixture in each cross involving a resistant and susceptible parent. Inoculum mixtures of the HV1 isolate with the remaining isolates resulted in lower disease grades compared to HV1 alone, suggesting an antagonistic or competitive effect.