Background: Striga hermonthica (Benth.) parasitism militates against increased maize production and productivity in savannas of sub-Saharan Africa (SSA). Identification of Striga resistance genes is important in developing genotypes with durable resistance. So far, there is only one report on the existence of QTL for Striga resistance on chromosome 6 of maize. The objective of this study was to identify genomic regions significantly associated with grain yield and other agronomic traits under artificial Striga field infestation. A panel of 132 early-maturing maize inbreds were phenotyped for key agronomic traits under Striga-infested and Striga-free conditions. The inbred lines were also genotyped using 47,440 DArTseq markers from which 7224 markers were retained for population structure analysis and genome-wide association study (GWAS). Results: The inbred lines were grouped into two major clusters based on structure analysis as well as the neighbor-joining hierarchical clustering. A total of 24 SNPs significantly associated with grain yield, Striga damage at 8 and 10 weeks after planting (WAP), ears per plant and ear aspect under Striga infestation were detected. Under Striga-free conditions, 11 SNPs significantly associated with grain yield, number of ears per plant and ear aspect were identified. Three markers physically located close to the putative genes GRMZM2G164743 (bin 10.05), GRMZM2G060216 (bin 3.06) and GRMZM2G103085 (bin 5.07) were detected, linked to grain yield, Striga damage at 8 and 10 WAP and number of ears per plant under Striga infestation, explaining 9 to 42% of the phenotypic variance. Furthermore, the S9_154,978,426 locus on chromosome 9 was found at 2.61 Mb close to the ZmCCD1 gene known to be associated with the reduction of strigolactone production in the maize roots. Conclusions: Presented in this study is the first report of the identification of significant loci on chromosomes 9 and 10 of maize that are closely linked to ZmCCD1 and amt5 genes, respectively and may be related to plant defense mechanisms against Striga parasitism. After validation, the identified loci could be targets for breeders for marker-assisted selection (MAS) to accelerate genetic enhancement of maize for Striga resistance in the tropics, particularly in SSA, where the parasitic weed is endemic.

Genotype main effect plus genotype-by-environment interaction (GGE) biplot produces a graphical display of results that facilitates a better understanding of complex genotype-by-environment interaction in multi-environment trials of breeding and agronomic experiments. However, the full potential and weaknesses of this powerful tool are not fully understood by breeders, agronomists, entomologists and pathologists. The objective of this paper was to review the usefulness of this statistical tool and enumerate some of its weaknesses. Its main application has so far been in the analysis of multi-environment data. It has been used to analyze the performance of crop cultivars under multiple stress environments, from which ideal cultivars, mega-environments, and core testing sites were identified. More recently, GGE biplot has been employed in genetic analysis of diallel data to estimate the combining abilities and identify heterotic groups among inbred parents. Genotype-by-trait biplot has also been utilized in trait profile analysis, and in identification of traits that are reliable for indirect selection of a target primary trait. Two major shortcomings of this tool are (i) failure to identify more than two distinct, contrasting groups in diallel studies and (ii) lack of statistical tests for most of its graphical displays. Other aspects of GGE biplot that need further study and development are (i) estimation of genetic variances, covariances, and heritability, including the analysis of data generated from North Carolina Designs I, II, and III as well as other genetic designs, considering their importance in plant breeding programs; (ii) analysis of Quantitative Trait Loci (QTL) data for proper understanding of the genetic constitution of each individual plant or line; and (iii) analysis of Genotype-by-pathogen or insect strain interaction data. Nevertheless, GGE biplot has helped greatly in the accurate analysis and interpretation of data from breeding and agronomic field evaluation experiments

Badu-Apraku, B. and Akinwale, R. O. 2011. Identification of early-maturing maize inbred lines based on multiple traits under drought and low N environments for hybrid development and population improvement. Can. J. Plant Sci. 91: 931–942. Drought and low nitrogen stresses are major limiting factors to maize (Zea mays L.) production and productivity in West and Central Africa. Studies were conducted from 2007 to 2009 at three locations in Nigeria under induced drought stress and low nitrogen conditions. The objective was to identify superior inbred lines for use as parents for hybrid production and for introgression into maize breeding populations. The inbreds TZEI 17, TZEI 13, TZEI 23, TZEI 2, TZEI 3, TZEI 22, TZEI 7, TZEI 11, and TZEI 8 were identified as the most promising parents under drought stress. Under low N, TZEI 7, TZEI 11, TZEI 2, TZEI 4, TZEI 10, TZEI 8, and TZEI 22 were selected. TZEI 11, TZEI 2, TZEI 8, and TZEI 22 had combined tolerance to drought stress and low N and could be used as germplasm sources for introgression of tolerance genes as well as for the development of drought and/or low-N-tolerant hybrids. Under drought stress, TZEI 17, TZEI 3, TZEI 23, and TZEI 13 were the closest to the ideal genotype, while TZEI 7, TZEI 2, and TZEI 11 were the closest under low-N conditions.

Drought, heat, and combined drought and heat are important abiotic stresses constraining the production and productivity of maize ( Zea mays L.) in sub-Saharan Africa (SSA). In the face of climate change, these stresses are likely to occur simultaneously and put at risk food and economic security in SSA. This review describes maize breeding activities conducted by the International Institute of Tropical Agriculture (IITA) in partnership with national scientists under the Drought Tolerant Maize for Africa (DTMA) and Stress Tolerant Maize for Africa (STMA) projects, which together sought to develop and deploy multiple stress tolerant hybrids, and open-pollinated varieties. Emphasis was on (i) developing a reliable methodology for screening maize for tolerance to drought stress (DS), heat stress (HS), and combined drought and heat stress (CDHS) using key secondary traits and grain yield, (ii) use of appropriate breeding techniques for tailoring maize for tolerance to DS, HS and CDHS, (iii) exploring diverse sources of germplasm for genetic enhancement of maize, (iv) extensive multilocational evaluation to identify genotypes with stable performance under the stresses, and (v) application of genomic tools to accelerate genetic gains in maize breeding at IITA. At IITA, the performance of maize hybrids under stresses of DS, HS and CDHS have been improved using conventional breeding techniques/procedures. These techniques/ procedures have led to accelerated genetic gains in yield that were 26–49% higher than the best commercial hybrid checks under CDHS and DS. Additive gene action has been consistently found to be more important than the non-additive among early maize under DS and CDHS while both the additive and non-additive have been reported to be important for the extra-early maize. The most reliable secondary traits for selecting for improved grain yield under the stresses include anthesis-silking interval, ears per plant, and plant and ear aspects. Several early and extra-early landraces have been identified as potential sources of tolerance to DS, HS, and CDHS. Several quantitative trait loci (QTLs) associated with grain yield and key secondary traits have been identified via genome-wide association studies in landraces and inbred lines. Those desirable QTLs, upon validation, could be invaluable for genomics-enabled breeding.

Northern corn leaf blight (NCLB) incited by the fungus Exserohilum turcicum is a foliar disease that significantly limits maize production and productivity in West and Central Africa (WCA), particularly in the mid-altitudes but during the last decade it has become a menace in lowland agro-ecologies. The most economical and environmentally friendly disease management strategy is the cultivation of maize varieties resistant or tolerant to NCLB. However, no early maturing (EM) and extra-early maturing (EEM) NCLB resistant varieties are commercially available in WCA. One hundred inbred lines each of EM and EEM derived from tropical maize germplasm were inoculated with a virulent isolate of E. turcicum at five locations in Nigeria during the 2017 and 2018 growing seasons. The objective of the study was to identify promising NCLB resistant lines and to investigate inter-relationships among the traits. Analysis of variance revealed highly significant genotype and genotype by environment (G×E) interactions for disease severity, grain yield (GYLD), and other agronomic traits. The average disease severity (TURC) values ranged from 1.9 to 5.8 and 2.9 to 5.7 for the EM and EEM inbred lines, respectively. The levels of reaction of the inbred lines to NCLB ranged from highly resistant to highly susceptible. Stepwise regression analysis showed that ears per plant, ear and plant aspects were significantly influenced by the disease scores. Ears per plant, ear and plant aspects, TURC and GYLD traits were employed to develop a base index (BI) for selecting NCL resistant inbred lines for hybrid development. TZEI 135 and TZEEI 1 were outstanding in GYLD and also had the highest positive BI values in the EM and EEM inbred lines, respectively. The identification of NCLB resistant lines in this study has set the premise for development of NCLB resistant hybrids for WCA as well as the improvement of tropical maize breeding populations for NCLB resistance