Plant breeding

Phaeosphaeria leaf spot (PLS) disease (causal agent Phaeosphaeria maydis (Henn.) Rane, Payak & Renfro) of maize is increasing in importance in sub-Saharan Africa (SSA). However, there is still limited information on the combining ability for disease resistance of the germplasm that are adapted to African environments. Evaluating combining ability effects and their interactions with the environment would provide valuable information that can be used in the development of cultivars that are resistant to PLS. This study was therefore conducted to determine the combining ability, gene action and the relationship between grain yield and PLS disease severity among selected tropical advanced maize inbred lines. Forty five F 1 hybrids were generated by crossing 10 inbred lines in a half diallel mating scheme. The 45 hybrids along with the ten inbred parents were evaluated in four environments, with two replications each between 2007 and 2009. General and specific combining ability (GCA and SCA) effects were highly significant (P ≤ 0.001) for PLS, grain yield and days to anthesis. GCA effects accounted for 66-90% and SCA effects for 10-34% of the variation in the hybrids for PLS resistance, grain yield and days to anthesis. This indicated predominance of additive over non-additive gene action for the three traits in these inbred lines. The resistant inbred lines to PLS were A1220-4, N3-2-3-3, CML312, MP18 and CML488. These lines had good combining ability for PLS resistance and contributed towards resistance in their crosses. In general, resistant hybrids involved a susceptible and a resistant parent, where at least one of the parents had a negative GCA effect. In addition, lines A1220-4 and CML312 contributed towards high yield and were late maturing. Inbred line CZL00009 conferred genes for early maturity. Linear regression analysis indicated that grain yield of maize was suppressed by about 250 kg ha −1 per each increase in PLS disease severity score, underscoring the need to control the disease. Significant (P < 0.01), negative correlations (r = −0.29 to −0.43) between grain yield and PLS severity were also detected. This showed the potential of PLS to reduce yield when favourable conditions for disease development are present. By and large, highly significant additive gene action implied that progress would be made through selection. Although non-additive effects were small (±10%), observation of dominance effects which were associated with reduced disease levels in some hybrids may be exploited in developing single cross maize hybrids among these inbreds when one parent is resistant.

Interspecific hybridization is an important technique used in improving rice populations by combining desired traits from different species. However, this could be difficult due to barriers to interspecific hybridization. The objective of this study was to determine the performance of different rice species based on seed set in an interspecific hybridization program. Five Oryza glaberrima genotypes and four interspecific inbred lines were used as female (seed) parents and two improved O. sativa and two interspecific inbred lines (NERICA 2 and NERICA 3) as male (pollen) parents to generate 36 cross combinations representing the North Carolina Design II mating scheme. Four groups of crosses were made: group A (O. glaberrima × O. sativa), group B (O. glaberrima × Interspecific), group C (Interspecific × O. sativa) and group D (Interspecific × Interspecific).

Nematodes cause substantial grain yield loss in susceptible maize (Zea mays L.) cultivars. This study was conducted to estimate general combining ability (GCA), specific combining ability (SCA) and genetic effects associated with nematode resistance in maize. The 30 F 1 hybrids generated from a 6 × 6 diallel and two local checks were evaluated in 2009 at three sites in Uganda. A split plot design was used with nematode treatments serving as whole plots and the hybrids as subplots but arranged in an 8 × 4 spatially adjusted alpha-lattice design. The experiment was replicated three times. Results showed GCA to be important for the reduction of P. zeae and Meloidogyne spp. densities and increase of root mass, with a contribution of 72 to 93% of the phenotypic variance. Inbreds MP709 and CML206 had the highest GCA for Pratylenchus zeae resistance, whereas for grain yield, it was CML444, CML312 and CML395 that were outstanding. The SCA influenced plant height and grain yield under nematode infestation, contributing 43 and 58% of the phenotypic variance, respectively. Observed reciprocal differences due to maternal effects also played a role in influencing the grain yield under nematode infestation. Overdominance genetic effects explained the non-additive variance recorded for the plant height, grain yield, number of root lesions, and P. zeae and Meloidogyne spp. densities under nematode infestation. The parents MP709, CML206, 5057, and CML444 contributed most of the dominant genes for the P. zeae resistance in all their crosses. The parent CML444 contributed most of the dominant genes for improved grain yield in all of its crosses. The high GCA effects among some parents support their utility in breeding of widely adapted nematode-resistant cultivars. The dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure.

Maize grey leaf spot (GLS) disease remains an important foliar disease in sub-Saharan Africa accounting for more than 25% yield losses in maize. Information on inheritance of GLS resistance of germplasm adapted to African environments is required in new sources being identified. Therefore, hybrids generated from a 10 × 10 half-diallel mating of tropical advanced maize inbred lines were evaluated in six environments to determine combining ability, genotype × environment interaction (G × E) and the impact of GLS disease on grain yield. General combining ability effects were highly significant and accounted for 72 and 68% of the variation for GLS resistance and grain yield, respectively. Significant specific combining ability effects associated with reduced disease levels were observed in some hybrids when one parent was resistant, and these may be exploited in developing single cross maize hybrids. Regression analysis showed a 260–320 kg ha−1 decrease in maize grain yield per each increase in GLS disease severity score, and significant associations (r = −0.31 to −0.60) were observed between grain yield and GLS severity scores. This showed the potential of GLS disease to reduce yield in susceptible varieties grown under favourable disease conditions, without control measures. Genotype and genotype × environment biplots and correlation analysis indicated that the significant G × E observed was not due to changes in hybrid ranking, implying absence of a significant crossover interaction. Therefore, predominance of additive gene effects imply that breeding progress for GLS disease resistance would be made through selection and this could be achieved at a few hot-spot sites, such as Baynesfield and Cedara locations in South Africa, and still deploy the resistant germplasm to other environments in which they are adapted.