Genetic studies on common bacteria blight caused by Xanthomonas campestris

Results of six experiments are reported herein. Seedling and adult plant resistance to common blight were not related and there was increasing susceptibility with maturity indicating that it was best to screen mature plants. Razor blade and multiple needle inoculation could be substituted for each other. Pod, stem and seed resistance were not related to foliar reactions of the same plants, this implying that these plant parts were under the control of different genes and needed to be tested separately. Both liquid and dry inoculum were good in screening seeds for resistance to common blight. The number of genes controlling resistance varied from one to four using Stanisfield's method and one too many with Mendelian genetics. The gene action was principally additive though some crosses showed epistatic gene action. There were low to high narrow sense heritability values the levels being affected by the method used to estimate, plus the type of cross. The additive gene action, few genes and high heritability values in some crosses meant that simple breeding methods could be used and selection carried out in early segregating generations while in other crosses this was not possible. There was an indication of the existence of resistance genes in the cytoplasm of some resistant sources depending on the method of analysis and the segregating generation. Thus it was better to use the resistant source as a female parent. Gain from selection up to the F,3 generation was very low. Crosses K20 x Jules, K20 x BAC6 and Jules x BAC6 involving Susceptible x Resistant and Susceptible x Intermediate parents were among the best specific combiners. The generation mean analysis was a more preferable method than diallel analysis in differentiating different components of epistatic gene action. On the whole the results on the genetic studies indicated that selection at V3 and pods could be made in early segregating generation due to additive gene action predominating while at R6 one needs to delay to later generations. To break the linkage between resistance and negative characters like low yields then back crosses, sib crosses or double crosses could be used. Seed resistance was important in preventing disease transmission by seed and quick cooking disease resistant plants could be selected from segregating generations.