Hussein, Shimelis.Dossa, Nanou Emeline.2025-06-242025-06-2420242025https://hdl.handle.net/10413/23776Doctoral Degree. University of KwaZulu-Natal, PietermaritzburgMaize (Zea mays L., 2n = 2x = 20) is a vital food security and economic crop in sub-Saharan Africa (SSA) and globally. In SSA maize production is challenged by an array of biotic and abiotic stresses. Two parasitic weeds belonging to the genus Striga, S. hermonthica (Del.) Benth (Sh) and S. asiatica (L.) Kuntze (Sa) causes marked yield losses varying from 10% to 100% in susceptible maize cultivars. Striga-resistant maize varieties released so far had partial or moderate resistance and were bred for Sh resistance only. There are therefore no commercially grown maize varieties with Sa resistance requiring to develop new-generation maize varieties with durable Sa and Sh resistance and wide adaptability using genetically diverse tropical and subtropical genetic resources and genomic resources. The overall objective of this study was therefore, to improve maize resistance to Sa and Sh by harnessing genetic diversity and identifying markers and genes for resistance breeding. The overall hypothesis of the study was that novel genetic resources, genetic markers and genes associated with Sa and Sh resistance could be identified for dual Striga resistance for maize breeding programs. The study had further five specific objectives: 1) To undertake a meta-analysis and provide a detailed comparison of the Striga control methods in the production of maize, sorghum, and the major millets as a guide to effective Striga management. 2) To assess the response of 130 tropical and sub-tropical African maize germplasm to Sh and Sa resistance and desirable agronomic traits and select promising genotypes. 3) To determine the genetic diversity of 130 tropical and sub-tropical maize inbred lines, hybrids, and open-pollinated varieties using phenotypic traits and single nucleotide polymorphism (SNP) markers to select Striga-resistant and complementary genotypes for breeding. 4) To determine the combining ability and gene action controlling grain yield and Striga resistance among single crosses of maize to select desirable hybrids with Sh and Sa resistance and promising agronomic traits. 5) To undertake a genome-wide association analysis of grain yield and Sh and Sa resistance among tropical and sub-tropical maize populations to identify putative genetic markers and genes for marker-assisted resistance breeding and gene pyramiding. In the first part of the study, a meta-analysis was conducted on already reported Striga control methods on the major cereal crops (i.e., maize, sorghum, and millets) using 66 research articles. The data collected included grain yield (GY), Striga emergence count (SEC), and Striga damage rating (SDR). The search showed mean yield for maize varieties with Striga-resistant genes at 2053.00 kg ha−1, ranging from 281.00 to 6260.00 kg ha−1, and a mean SDR of 4.70, varying from 2.00 to 7.00. Likewise, sorghum varieties with Striga resistance genes achieved greater GY with a mean yield response of 1738.00 kg ha−1, ranging from 850.00 to 2162.00 kg ha−1. A relatively low GY was achieved in maize and sorghum production when deploying integrated Striga management (ISM) (e.g., cultural control + host resistance, and host resistance + chemical herbicides) and chemical Striga control. The outcome of this part of the study was that SDR is the best selection criterion for improving GY performance in maize, while SEC and SDR were the parameters of choice in sorghum selection programs for better GY under Striga infestation. The meta-analysis revealed that host resistance is the most effective method for controlling Striga infestation and boosting GY in maize and sorghum. The second part of the study focused on screening 130 tropical and sub-tropical maize germplasms, including checks, in a controlled environment for their reaction to Sh and Sa infestations using a 13×10 alpha lattice design with two replications over two seasons. The following data were collected on maize: days to 50% silking (DS), days to anthesis (DA), anthesis-silking interval (ASI), plant height (PLHT), ear height (EHT), Root lodging (RL), the number of ears per plant (EPP), husk cover (HUSK), ear aspect (EASP), and grain yield per plant (GY/plant). Striga parameters included the number of emerged Sa and Sh plants 8 and 10 weeks after planting, denoted as SEC8 and SEC10, and host plant damage by Striga 8 and 10 weeks after planting, designated as SDR8 and SDR10. The mean yield of maize and Striga par were 3.35 and 3.07, respectively. Under Sh-infested conditions, SEC8 and SEC10 mean values were 3.66 and 3.77, respectively, while the SDR8 and SDR10 values were 5.25 and 2.75 respectively. The results suggested that dual resistance to the two Striga species exists in some tropical and sub-tropical maize lines. The study selected genotypes CML440, CML566, CML540, CML539, CLHP0343, CLHP0326, TZISTR1248, TZSTRI115, TZISTR25, TZISTR1205, TZSTRI113, TZISTR1119, TZISTR1174 and the OPVs B.King/1421, Shesha/1421, ZM1421, DTSTR-WSYN13, DTSTR-YSYN14, and 2*TZECOMP3DT/WhiteDTSTRSYN) C2 with dual resistance to Sa and Sh. These genotypes are suitable for use as parents in developing high-performing maize varieties with Striga resistance and improved grain yield. The third part of the study assessed the genetic diversity of 130 tropical and sub-tropical maize inbred lines, hybrids, and open-pollinated varieties using Striga resistance and agronomic traits, and SNP markers. The SNP markers demonstrated that the test genotypes had an average gene diversity of 0.34 and a polymorphic information content of 0.44, indicating significant phenotypic variation. Significant variation was recorded within populations (85%) compared to between populations using the analysis of molecular variance. The structure analysis allocated the test genotypes into eight major clusters (K = 8) in concordance with the principal coordinate analysis (PCoA). The following genetically distant inbred lines were selected, displaying good agronomic performance and Sa and Sh resistance: CML540, TZISTR25, TZISTR1248, CLHP0303, TZISTR1174, TZSTRI113, TZDEEI50, TZSTRI115, CML539, TZISTR1015, CZL99017, CML451, CML566, CLHP0343 and CML440. The new selections will now facilitate the breeding of maize varieties with Striga resistance and market-preferred traits. In the fourth part of the study, a combining ability analysis was undertaken to determine the mode of gene action regulating Sa and Sh resistance and to select good combiner parental maize lines for hybrid breeding. Four preliminarily selected tropical high-yielding and Sh-resistant testers and eight sub-tropical lines with Sa resistance were crossed using a line-by-tester mating design, and 32 single cross hybrids were generated. The crosses and their parents were evaluated under field and controlled environments during the 2023/2024 growing season using a 7 x 6 alpha lattice design with two replications. Combined analysis of variance revealed a significant (p<0.05) effect of the crosses on grain yield (GY), related agronomic traits, Striga emergence counts, and Striga damage rating 8 and 10 weeks after sowing. The ratio of the general combining ability effect (SCA) and the specific combining ability effect (SCA) was less than one for all the traits, indicating the predominance of non-additive genetic effects in trait inheritance and signifying the value of hybrid breeding. The best general combiner tester was TZISTR1248 in the Sa-infested environment, while tester TZISTR1174 was noteworthy under Sh environment. Lines CML540 and CLHP0343 were the best combiners in Sa environment, while CZL99017, CML566, CML540, and CLHP0343 were promising in Sh environment and CML540 was the best general combiner in all test environments. The crosses CML540 x TZISTR1174, CML540 x TZDEEI50, and CML539 x TZISTR1174 exhibited high yields, significant SCA effects, and high heterosis for GY in Sa environment. Whereas, in Sh environment, cross CML440 x TZDEEI50 had the best GCA effect and heterosis for GY. Crosses CML451 x TZISTR1174, CML539 x TZISTR1174, CML440 x TZDEEI50, CML566 x TZDEEI50, CZL99017 x TZISTR1248, and CML539 x TZISTR1248 were relatively the best specific combiners for GY in both Sa and Sh environments. The selected lines and testers and the new experimental hybrids are recommended for multi-environment evaluation in Sa and Sh-prone agroecologies to enhance grain yield and Striga resistance. In the fifth final part of the study, a genome-wide association analysis of grain yield and Sh and Sa resistance among tropical and sub-tropical maize populations was undertaken to identify putative genetic markers and genes for resistance breeding. The test genotypes were profiled for GY, SEC8, SEC10, SDR8, and SDR10. Population structure analysis and genome-wide association mapping were undertaken based on 16,000 single nucleotide polymorphism (SNP) markers using the Diversity Array Technology Sequencing platform. The genome-wide association study identified 50 significant loci associated with Sh resistance and 22 significant loci linked to Sa resistance, corresponding to 39 and 19 candidate genes, respectively. No significant loci were found associated with dual resistance, suggesting that breeding maize must be specific for resistance to each Striga species using germplasm adapted to the endemic region of each parasite. Overall, the study finally revealed a novel result that host resistance is the most effective method for controlling Striga infestation and boosting GY despite that research institutions advocate integrated Striga management. Promising genotypes with Sa and Sh resistance were selected, and some tropical and sub-tropical genotypes showed dual resistance. Suitable parental lines and testers and new experimental hybrids were selected for Sa and Sh resistance breeding in SSA. The new selections could be explored for future Striga resistance breeding and the development of new varieties. Significant loci associated with Sh and Sa resistance with their corresponding genes were detected and could be used to facilitate selection for Sh and Sa resistance and GY in tropical and sub-tropical maize genetic resources.enGrain yield.Maize.Resistance breeding.Striga asiatica.Striga hermonthica.Thesis