RACHEL CANN
GENETIC STUDIES OFiRESISTANCE TOiMAIZE STREAK VIRUS DISEASE IN MAIZE (Zea mays L.)
ABSTRACT
Maize Streak Disease (MSD) is one of the most significant viral diseases that affect maize production in Ghana. Breeding of maize hybrids that are resistant to MSD is the most effective and sustainable way of controlling the disease. The objectives of the study were to i) screen maize inbred lines for resistance to MSV using SSR markers, ii) evaluate maize inbred lines for responses to MSD under artificial inoculation and natural infection in hot spot environments, iii) assign inbred lines to heterotic groups, determine heterosis and combining ability for grain yield and other traits, iv) investigate the G X E interaction for grain yield among newly developed maize hybrids. Molecular markers were employed to screen and select maize lines that were resistant or susceptible to MSD. The lines were further characterized under artificial and natural inoculations. Three resistant testers and 18 susceptible but promising lines were selected and crossed using line x tester mating design to develop 54ihybrids. The hybrids, parents and checks were evaluated across five environments to assess the general combining ability of the parents and specific combining ability of the hybrids. The hybrids were evaluated across five environments to assess their yield stability across the environments. Sixty-eight (68%) of the inbred lines screened with SSR markers were resistant. Sixty inbred lines were also evaluated in field trials to validate their response to MSD under artificial and natural inoculation environments usingi10 x 6 alpha lattice design with two replications. Field experiments were established in 2019 during the major and minor seasons at Kwadaso and Wenchi under artificial and natural inoculations. Analyses of variance of quantitative traits revealed significant variation among the inbred lines. Grain yield correlated negatively to virus incidence and virus severity but correlated positively to ear diameter and 1000 grain weight. Principal component analysis revealed that 69.5% of the total variation was contributed by four principal components. Cluster analysis grouped the inbred lines into two main clusters using the qualitative traits. Eighteen selected inbred lines were crossed to three testers to generate 54 hybrids. Parents, hybrids and six standard checks were evaluated using 10 x 6 alpha lattice design with two replications at Wenchi, Branam and Kwadaso in 2020 during the major and minor seasons under natural and artificial inoculation. General combining ability of parents, specific combining ability of hybrids, and heterosis of grain yield and other agronomic traits were estimated. Under natural inoculation environment, significant mean square values for GCA (line) were observed for all traits except, anthesisisilking interval, plant height, ear diameter, 1000igrain weight and husk cover. TZIL25 and 9006 were identified to be resistant to MSD across the two locations irrespective of the method of inoculation. Significant mean square values were observed for GCA (tester) for virus severity, virus incidence and 1000 grain weight. Significant mean square values for SCA were observed for days to anthesis, days to silking and 1000 grain weight. Under artificial inoculation environment, significant mean square values were observed for GCA (line) for all traits except ear height however, GCA (tester) was not significant for most of the traits. Significant mean square values for SCA were observed for plant height, ear diameter, cob width and husk cover. Significant mean square values for SCA were observed for days to anthesis, days to silking and 1000 grain weight. The preponderance of GCA mean squares over SCA mean squares implied that additive gene action was more important than non-additive gene action for most of the traits and that GCA was the major contributing factor for the differences among hybrids. GGE biplot was used to analyse the genotype by environment interactions. This was done to detect maize genotypes that were stable with high yield performance under MSD hot spot and artificial inoculation environments. Ten high yielding hybrids and two stable hybrids were identified. These hybrids could be tested extensively in multi-location trials for release and promoted for adoption.