Effect of Genetic Divergence of Striga hermonthica (Delile) Benth.–Resistant Maize Inbred Lines on Heterosis and Hybrid Performance under Parasite Pressure

Striga hermonthica (Delile) Benth. is a parasitic weed damaging maize and other cereals in Africa. Although S. hermonthica–resistant inbred lines were developed from diverse sources, the effect of their diversity on heterosis under parasite pressure has not been investigated. Sixty-four hybrids were produced from 16 resistant and four susceptible inbreds using a factorial mating in sets crossing scheme. The hybrids and their parents were evaluated in separate trials with and without S. hermonthica infestation at four environments in Nigeria. Genotypic differences among both inbreds and hybrids were significant for all measured traits under infestation. The hybrids displayed a broad range of midparent heterosis (MPH) for most traits measured under S. hermonthica–infested and noninfested conditions. Mean grain yield and agronomic performance of resistant × resistant cross hybrids were superior to that of susceptible × resistant cross hybrids in the presence of S. hermonthica but not in the absence of the parasite. Correlations between midparent values and hybrid means were large and positive for grain yield but were small for plant height and days to silking. Marker-based genetic distances of parental lines were not correlated with MPH of all traits recorded under S. hermonthica infestation. Selection for inbreds with greater levels of resistance to S. hermonthica appears to be more effective for developing resistant hybrids than selection of parental pairs based on genetic distance alone.