DR. ALLEN OPPONG

DEVELOPMENT OF TOPCROSS HYBRID MAIZE (Zea mays L.) FOR YIELD AND RESISTANCE TO MAIZE STREAK VIRUS DISEASE, ABSTRACT

Studies were conducted to explore the potential of utilizing Ghanaian maize landraces for improved grain yield and resistance to the maize streak virus disease. Farmer perceptions of the maize streak virus disease as well as constraints to the adoption of improved varieties and farmers’ continuous reliance on local landraces or farmer varieties and production constraints were investigated. Genomic characterization of Maize streak virus (MSV) strains found in the forest and transitions zones of Ghana were also undertaken to identify virulent strains of the virus that can be used to screen novel genotypes with adequate resistance to the disease.

Genetic relationships among selected Ghanaian maize landraces and “exotic” germplasm were analyzed using SSR markers. Studies on general combining ability (GCA) and specific combining ability (SCA) on yield and related traits as well as MSV disease incidence and severity were conducted on selected local landraces and some CIMMYT and IITA inbred line testers using the line by tester approach. Percentage heterosis and heterotic groups were assessed in the local germplasm. The yield stability of selected topcross hybrids generated together with their landrace parents were assessed using AMMI and yield stability index in six environments.

Sixty four (64%) of respondents cultivated improved OPVs released by the research institutes and about 36% and 3% depend on landraces and hybrids respectively. Majority of the farmers preferred cultivars with slender cobs and lot of grains. Constraints identified by the farmers included climate change, poor road infrastructure, lack of farming inputs and cheating by middlemen. Majority of farmers interviewed had knowledge of the maize streak virus disease. Maize streak virus disease was mentioned as a seasonal disease which affects about 20% of their yields. Variants of MSV A1 identified from the disease specimen also aligned closely with those that have been detected in West Africa. The UPGMA dendrogram showed one big cluster with two outliers. However, eight minor clusters and nine outliers were found, if a perpendicular line is drawn from 0.22 genetic similarity, whilst Bayesian analysis indicated four sub populations and a mixed group. Significantly high mean yields were obtained for some of the topcross hybrids compared with their parents and the local checks with as much as 111% heterosis being observed for some of topcross hybrids over the best parent yield. Heterotic groups were assigned to 11 of the landraces (farmer varieties) whilst the remaining 5 could not be assigned. Significantly positive general combining abilities (GCA) effects with respect to yield were observed for lines LA30, LA3 and LA80, while significantly positive specific combining ability effects were observed for crosses involving lines LA558, LA76, LA80, LA457 and LA3. Testers CML442, CML444 and TZEI17 GCA effects significantly contributed positively to yield increase but significantly negative to MSV disease incidence and severity. Yield stability index ranked most of the topcross hybrids as high yielding and stable across environments.
This work has demonstrated the potential of utilizing Ghanaian maize landraces for improved yield and resistance to the Maize streak virus disease. It is recommended that inbred lines are developed from some of the promising landraces identified which can be used to produce single or double cross hybrids with yields and Maize streak virus disease resistance better than what was obtained in this study for wider adoption.