Bean stem maggot resistance in root rot resistant and drought tolerant breeding lines
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Bean stem maggot (Ophiomyia spp., Diptera, Agromyzidae) is a major field insect pest limiting common bean production in Uganda. The damage caused by bean stem maggot (BSM) is more severe under drought and the pest usually co-occurs with root rot pathogens and provides entry points for them. Due to the interaction between BSM, root rot causing pathogens and drought, varieties with multiple resistances to BSM, drought and root rots are probably the best option under farmer’s field conditions. Even though, some BSM resistant lines exist, the resistance is not effective in the presence of root rots and drought stress. Similarly, sources of root rots resistance and drought tolerance do exist but information on their levels of resistance to BSM and gene action in these back grounds is lacking. This research was therefore carried out to identify sources of BSM resistance among drought tolerant and root rot resistant lines and determine the gene action governing BSM resistance. To identify sources of resistance, one hundred genotypes were screened under natural field infestation and in the screen house for BSM and root rots resistance respectively. Results showed that variability in BSM and root rots severity due to genotype was highly significant for all the traits measured. Regarding BSM resistance, most lines (96%) performed better than the susceptible check but only seven outperformed the resistant check, G21212. Thirty-seven lines had dual resistance to Fusarium and Pythium root rots. Among the drought tolerant and root rot resistant lines evaluated in this study, eight lines were identified as good sources of BSM resistance. To determine the nature of gene action controlling BSM resistance, three BSM susceptible lines with resistance to root rots and tolerant to drought and three BSM resistant parents were crossed in a 6 x 6 half-diallel mating design. The resultant F2 progeny was screened for reaction to BSM in the field under natural infestation. Data on plant emergence, plant mortality, stem damage; number of larvae and pupae in the stems was collected. Both general combining ability (GCA) and specific combining ability (SCA) were insignificant for all traits studied. Basing on the GCA and SCA estimates, there was a strong environmental effect and an involvement of both the additive and non-additive in conferring resistance to BSM. Low general predictability ratios (0.00 - 0.26) were noted for all the traits apart from plant mortality. Narrow sense heritability estimates were low for all the traits whereas broad sense heritability estimates were low for the number of pupae, moderate for stem damage and plant mortality and high for the number of larvae. Therefore, Heritability of BSM resistance is low basing on stem damage, number of larvae and pupae in the stems however a high heritability based on plant mortality was noted. Results of this study revealed that lines 232, 164, BFS14, G14, G79, G60, ALB8 and G21212 are resistant to BSM. Plant mortality as a basis of selection and methods that involve selection for BSM resistance in later generations are highly recommended to plant breeders.