Breeding maize for resistance to the fall armyworm (Spodoptera Frugiperda J.E. Smith), improved yield and yield-related traits.

Abstract

Maize production and productivity in sub-Sub-Saharan Africa (SSA), including Zambia, has been severely threatened by the recent arrival of the fall armyworm (FAW) (Spodoptera frugiperda J.E. Smith). Several strategies have been proposed to control FAW. Integrated pest management (IPM), emphasizing host-plant resistance, has been identified as the most sustainable approach. However, validated and locally-adapted FAW-resistant maize cultivars have not yet been developed and deployed in sub-Saharan Africa. The aim of this research was to develop FAW resistant, farmer-preferred and locally adapted maize varieties. The specific objectives of this study were: (1) to identify farmers' maize production constraints and preferred traits, and to assess farmers' perceptions of the impact of FAW on maize production and productivity in Zambia; (2) to screen a diverse population of maize genotypes for FAW resistance, yield, and yield-related traits by phenotyping and genotyping, to select the most promising genotypes for crossing; (3) to optimise the methods for laboratory rearing of FAW, and to conduct artificial screening of promising maize genotypes under controlled conditions and artificial FAW infestation; (4) to determine the level of genetic diversity for agronomic and FAW-related traits among the test population of maize genotypes using SNP markers complemented by phenotypic information to identify suitable parents for developing FAW-resistant breeding populations; and (5) to determine the nature of gene action conditioning FAW resistance, yield, and yield-related traits through combining ability analysis, and to identify the most promising crosses for continued evaluation in FAW resistance breeding. In the first study, participatory rural appraisal (PRA) surveys were conducted in two FAW affected districts in Zambia in 2017 and 2018, using semi-structured questionnaires, preference ranking and focus group discussions. The high cost of fertilizers, the limited availability of agricultural lands, insect pests, and drought stress were reported by 73, 55, 38 and 36.6% of the respondents, respectively, as the main production constraints. There were significant differences (X2 = 12.415; p = 0.002) in the severity of FAW infestation between the two surveyed districts in 2017. Farmer-preferred traits of maize were insect pest resistance, early maturity, drought tolerance and market price of the grain. FAW resistance, drought tolerance and grain yield performance are the key drivers for maize variety development and deployment in Zambia. In the second study, two sets of diverse maize germplasm were assessed for FAW resistance and desirable yield-related traits in a FAW-affected area in Zambia. Set I and Set II, containing 60 and 253 maize genotypes, respectively, were selected, based on their agronomic potential and adaptability. Highly significant differences (P < 0.001) were detected among the test genotypes for FAW leaf damage (FLD), FAW cob damage (FCD) and agronomic traits. The lowest levels of FLD and FCD were 8.87% and 5.36%, recorded for genotypes CML304-B and CML442, respectively. Five principal components (PCs) accounted for ≥80% of the total variation associated with reduced anthesis-silking interval (ASI), plant height, FLD and FCD, desirable ear aspect and grain yield. Genotypes such as Pool 16 and ZM 7114 from Set I, and CZL1310c, CML444-B, CZL15220 and TL1512847 from Set II had low mean FCD and FLD values, suggesting higher levels of FAW resistance. Grain yield was negatively correlated with mean FLD (r = 0.18, p <0.05), and FCD (r = 0.15, p < 0.05). Promising maize genotypes, including CZL1310c, CML444-B, CZL15220, TL1512847 and CML491, were selected for their low mean FLD and FCD, earliness to flowering and high grain yield potential. These genotypes should be useful in developing tropical and sub-tropical maize breeding populations with partial FAW resistance and yield gains. The third study optimised laboratory rearing and artificial inoculation of FAW onto maize plants under controlled conditions. Field-collected FAW egg masses and larvae were used to mass-produce fresh colonies of the larvae and to evaluate 63 maize genotypes for FAW resistance. The study enabled an understanding of the salient features of FAW growth and development under local environments to implement integrated FAW management strategies. Test genotypes had differential reactions to FAW infestation under controlled conditions. Several genotypes, including CML545-B, CZL1310c, VL050120, CZL16095, EBL169550, ZM4236, MM501 and Pool 16, exhibited considerable FAW resistance at the seedling and leaf-whorl growth stages, and were selected for resistance breeding. The study established a standardised laboratory and screen house-based protocol for mass rearing and artificial infestation of FAW to screen maize genotypes for resistance breeding programs in Zambia or other sub-Saharan Africa countries. The fourth study determined the genetic diversity of 59 maize genotypes of diverse genetic backgrounds with variable resistance to FAW, using phenotypic traits and SNP-based DArT markers. The test genotypes were profiled using agro-morphological traits, FAW damage parameters, and Diversity Array Technology Sequencing-derived single nucleotide polymorphism (SNP) markers. Significant (p < 0.001) differences were observed among the genotypes for 13 phenotypic traits, with their phenotypic coefficient of variation ranging from 2.19 to 51.79%. Notable phenotypic variation was observed for ear position, grain yield, and FAW-induced leaf and cob damage. The mean gene diversity and polymorphic information content were 0.29 and 0.23, respectively, reflecting a moderate level of genetic variation among the test genotypes when assessed using SNP markers. Analysis of molecular variance revealed greater genetic variance within a population than between populations. Population structure and cluster analysis grouped the test genotypes into two main clusters. Three genetically divergent, open pollinated varieties were selected for their favourable agronomic performance and FAW resistance for population improvement or hybrid breeding: Pool 16, ZM 4236 and ZM 7114. The genetic diversity detected within and among the tested populations will facilitate the breeding of maize varieties incorporating farmer-preferred agronomic traits and FAW resistance in Zambia and related agro-ecologies. The fifth study investigated the combining ability effects and inheritance of FAW resistance and agronomic traits in maize genotypes selected for breeding. A line × tester mating design was used and 60 experimental hybrids were generated and field-evaluated in three FAW hotspot locations in Zambia. Both the general and specific combining ability effects were significant (p<0.05) for the assessed traits. Non-additive genetic effects were more important for the inheritance of grain yield and FAW-inflicted leaf and cob damage, suggesting that heterosis breeding would be the best strategy for yield gains. The narrow sense heritability (h2) estimates for agronomic and FAW-related traits ranged from 0.14 to 0.47 and 0.37 to 0.49, respectively. The experimental hybrids CML346/EBL16469, ZM4236/CML545-B, CML346/CZL1310c, CML334/EBL173782, CML545-B/EBL169550 were among those selected with favourable specific combining ability estimates for greater grain yield, reduced days-to-50% anthesis, days-to-50% silking, FAW leaf and cob damage resistance, respectively. The selected experimental hybrids are recommended for further evaluation and breeding. Overall, the study developed and optimized the techniques for the artificial rearing and infestation of FAW on maize under controlled conditions. Promising inbred lines and new FAW resistant experimental maize hybrids were developed involving landrace varieties and donor parents sourced from the International Maize and Wheat Improvement Center (CIMMYT). This study contributes to the development of FAW resistant maize varieties in Zambia and SSA.