Genetic resistance to bean stem maggot (O.spencerella) in Ugandan bean genotypes

Common bean (Phaseolus vulgaris.L) production has been greatly affected by the bean stem maggot (BSM), an insect pest which exists in 3 species namely O.spencerella, O.phaseoli and O.centrocematis. Among the 3 species, O.spencerella and O.phaseoli are of major economic importance and cause up to 100% yield loss. In Uganda, the breeding program seeks to avert this problem by developing resistant varieties for the Ugandan farmers. To achieve this, the breeders require information to guide breeding decisions. The objectives of this study were to determine the level of resistance to the bean stem maggot (O.spencerella) in the exotic and local Ugandan bean genotypes, determine the nature of inheritance of BSM (O.spencerella) and heritability of resistance to BSM (O.spencerella). Genetic variability for resistance to the BSM was determined under natural infestation where the 32 genotypes which included 4 Ugandan varieties and 28 introductions from Malawi that were reported to be resistant to the pest were evaluated for percent plant mortality, number of ovipunctures, number of pupae and stem damage scored on a scale of 1-9. Significant effects were obtained for number of ovipunctures (P≤ 0.05), stem damage (P≤ 0.01) and number of pupae in the stem (P≤ 0.001) suggesting that the genotypes had varying levels of resistance to the BSM under Uganda environments. Several of the screened parents were resistant and moderately resistant with very few being susceptible. Some of the resistant sources that can be utilized include Line 19, 51, 12, 136, 6 and G 21212. Based on the results of the screening study, 16 Malawi genotypes were crossed to NABE 4, NABE 15, NABE 16 and NABE 17 using a NCD II without reciprocals to generate information about the inheritance of resistance to BSM. The F1 seed were advanced to F2 and the resultant progeny used for the inheritance evaluation. The analysis of variance showed that the general combining ability (GCA) for the male and female parents was not significant for all resistance parameters. Similarly, the specific combining ability (SCA) was also not significant. Both the narrow sense and broad sense coefficient of genetic determination values were low for all resistance parameters which included percent mortality, number of ovipunctures, number of pupae in stem and stem damage. The NS-CGD was in the range of 0.00 to 0.11 and BS-CGD in the range of 0.00 to 0.42. Such estimates suggest that non-additive gene action is more important compared to the additive gene action in transmitting the genes that confer resistance to the BSM (O. spencerella). The results of the general predictability ratio (Bakers ratio) for the resistance parameters showed that the SCA effects were much higher compared to the GCA effects in conferring resistance to the F2 progeny as the ratio was in the range of 0.00 to 0.37. The NS-CGD and BS-CGD for yield were 0.475 and 0.577 respectively which indicated moderate heritability with GCA effect estimates being higher than the SCA estimates as shown from the general predictability ratio of 83% supporting the predominance of additive gene action in conditioning yield potential. The evaluation of heritability was also conducted under natural bean fly infestation. The F2 seed derived from the hybridization of the local and Malawi genotypes were advanced to F3 and used for BSM resistance evaluation. The heritability estimates for resistance to BSM were obtained using mid- parent offspring regression. Both the F2 and F3 progeny data which were regressed to the mid parent scores revealed that the F2 and F3 had heritability estimates in the range of 1% - 97% and 4% -37% for the different resistance parameters. This suggests that for some parameters with high heritability estimates, selection should be done early unlike those with low heritability where selection is recommended in later generations.