Inheritance of resistance to sclerotium root rot (Sclerotium rolfsii Sacc.) in common bean (Phaseolus vulgaris L.) in Uganda

Abstract

Sclerotium root rot caused by Sclerotium rolfsii Sacc. is one of the major soil-borne diseases of common bean in Uganda and many other countries. The disease is very aggressive causing pre-emergence damping off. It affects roots causing lesions followed by total death of the plant. An incidence and severity of 100% and 84% respectively, has been reported in the common bean production environments in Uganda. The use of resistant cultivars is reported as the most efficient management strategy against root rot diseases. A few breeding lines showing good levels of resistance have been identified, however, currently all released varieties in Uganda lack resistance to the disease and mode of inheritance of resistance to Sclerotium rolfsii Sacc. has not yet been studied in common bean. The objectives of this study were to enhance breeding for Sclerotium root rot resistance through validating the resistance in the identified breeding lines and identify new sources, and determine the inheritance pattern of resistance to Sclerotium root rot. A total of 72genotypes with two checks (K132 and KWP 9) were evaluated under screen house at NaCRRI (2017A & 2017B) with 9 x10 alpha-lattice design arrangement by three replications. Each genotype was planted in a wooden tray containing pre-sterilized soil inoculated with matured LWE 336-1 Sclerotium isolate. The checks were placed randomly on a plot in the trays. Germination and disease severity were assessed 14 and 28 days after planting (DAP), respectively. The result showed a significant (P<0.001) difference among genotypes both for germination percentage and disease severity, suggesting that substantial genetic variation among the evaluated genotypes. The breeding lines: KWP 12, KWP 17, KWP 9, and ALB 171 were confirmed to be resistance and RRA 41 was identified as a new source of resistance. These genotypes were therefore recommended for further utilization in common bean breeding program to improve Sclerotium root rot resistance in Ugandan susceptible varieties. To determine the inheritance pattern, four resistant (KWP 9, ALB 171, KWP 12, and KWP 17) and three susceptible (K132, NABE 14, and NABE 15) genotypes were selected and crossed in a full diallel mating design. The F1and their reciprocal were advanced to the F2 generation. Disease evaluation was done for F2 and their reciprocal, together with their parents against LWE 336-1 Sclerotium isolate with 7x7 alpha-lattice design arrangement by three replications under screen house. The F2 segregation pattern revealed that resistance to Sclerotium root rot in common bean had a qualitative inheritance with one to three major genes and epistatic effects. Significant general and specific combining abilities were observed both for germination percentage and disease severity, indicating that both additive and non-additive gene effects were important for Sclerotium root rot resistance in the common bean genotypes utilized for the study. The relative importance of additive to non-additive was moderate (0.60) for both disease severity and germination percentage, suggesting that about 60% of observed variation was explained by additive gene effects. The estimated broad sense coefficient of genetic determination was high 0.78 and 0.65, and narrow sense was moderate (0.47) and low (0.33) for disease severity and germination percentage, respectively, suggesting that selection should be done at later generation. The resistant parents KWP 9, ALB 171 and KWP 17 had significant desirable GCA effects, suggesting that they were good at transmitting the resistance when crossed with other parents. These parents, therefore, are potential sources for resistance to Sclerotium root rot disease and can be used for introgressing resistance in to susceptible Ugandan market class varieties.