GENETIC ANALYSIS OF AGRONOMIC TRAITS AND TOLERANCE TO LATE LEAF SPOT IN GROUNDNUT (ARACHIS HYPOGAEA L.)
Late leaf spot (LLS) Phaeoisariopsis personata (Berk. and Curtis) Deighton is one of the most important foliar diseases of groundnut worldwide including Nigeria. Repeated application of fungicide could contribute to greater production cost and environmental pollution. The deployment of resistant cultivars offers a better option for the management of the disease in groundnut. Information on the genetic control of resistant to LLS will aid in the development of appropriate breeding methodology that will result in the development of resistant genotypes with high yielding potentials. The objectives of this study were to determine the mode of gene action controlling LLS in some groundnut genotypes, estimate heritability and to determine the degree of association between yield components and LLS resistance. Nine parental groundnut genotypes comprising of five LLS resistant (ICGV 12991, ICGV 7878, FDR-F7 82, FDR-F7 67 and FDR-F7 61) and four LLS susceptible (SAMNUT 23, SAMNUT 24, SAMNUT 25 and SAMNUT 26) genotypes obtained from IAR were used to generate F1 hybrids using 4 x 5 North Carolina design II mating design. The 20F1s and the parental genotypes were evaluated for LLS in a randomized complete block design with three replications at the screen house of the Department of Plant Science under artificial inoculation. High relative humidity was maintained by covering the plants with polyethylene bags for 24 hours before and after inoculation. Data were collected on pod yield, disease severity, disease incidence and other agronomic traits. The results of the analysis of variance indicated highly significant (P 0.01) differences for the traits of the genotypes evaluated which suggest that progress could be made through selection. High heritability coupled with low GAM for LLS tolerance traits, suggesting large environmental influence on the traits and predominant role of additive gene effect. Both narrow and broad sense heritability were high for the LLS tolerance traits, suggesting the role of additive gene action, hence mass selection could be effective to exploit any additive gene action. Highly significant negative GCA effects were recorded for disease incidence in SAMNUT 25 (-0.05), indicating the genotype is a good general combiner for LLS disease incidence and SAMNUT 26 (-5.90) is a good general combiner for LLS disease severity. ICGV 12991 (-12.00), FDR-F7 67 (-.68) and FDR-F7 61 (-2.23) are good general combiner for LLS disease tolerance (disease incidence and severity) Among the crosses, significant and negative SCA effects were obtained for most of the crosses SAMNUT 24 x FDR-F7 67 and SAMNUT 24 x FDR-F7 61 for LLS tolerance (disease incidence and severity), indicating that they are good specific combiners for LLS tolerance. The ratio of the GCA and SCA variances indicated the preponderance of SCA variance over GCA variance for disease incidence and severity, indicating the role of non-additive gene effect and it may be due to difference in genotypes used as parents. The female parents contributed more than the male parents, suggesting the maternal effects and that when developing breeding population to LLS tolerance, the choice of maternal parent is very important. Significant negative correlation was observed between diseases (incidence and severity) and number of seed per pod as well as 100-seed weight which implies the selection for low disease incidence and severity would increase the number of seed per pod and 100-seed weight.