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Development of maize populations resistant to stem borers depends largely on the existence of useful genes or alleles, which can combine to confer resistance to progenies. Such genes are often available in areas of stress, having been responsible for the survival of such crops over the years. Pink stem borer, Sesamia calamistis (Hampson, Noctuidae) and sugarcane borer, Eldana saccharina (Walker, Pyralidae) are endemic in southeastern Nigeria. Damages caused by the larvae of these moths are more prevalent during the second planting season (August-November). Genetic diversity for a range of agronomic and resistance attributes within 209 local maize collections from southeastern Nigeria and 3 improved check varieties were investigated in field trials in randomised complete block design (RCBD) with two replications across three environments. Data collected from the evaluations were subjected to both uni- and multivariate statistics. Furthermore, four traits namely, leaf feeding, ear damage, shoot breakage and yield were used from across three environments to construct a selection index. The multivariate analysis on the plant attributes, using canonical discriminant analysis, revealed the agronomic and borer damage parameters that contributed significantly to the total variation observed in different environments. Out of the four canonical discriminant functions obtained, two had significant (P=0.05) eigenvalues accounting for over 98 % of the total variation. The first canonical function was mainly associated with yield while the second was associated with the borer damage attributes. Rank summation index (RSI) used to rank the entries for resistance to stem borers identified 11 genotypes representing top 5 % of the total as resistant. In the second experiment the 11 genotypes and their hybrids, made in a diallel fashion were evaluated for agronomic and borer damage attributes in seven environments in RCBD with three replications. Data collected were subjected to analysis of variance and those found significant (P=0.05) were further subjected to diallel analysis using
Griffing’s method 2 model 1 for fixed effects. Significant GCA and SCA effects were obtained for most of the traits studied in the various environments and in the pooled environment thus indicating that additive and non-additive gene effects were involved in the expressions of the traits studied. However, in a few cases, only GCA or SCA was important thus indicating the relative importance of the genetic component of the variance. The assessment of the agronomic and borer damage attributes of the parents and the crosses indicate that the variety crosses were not superior to the parents in most of the traits. The significant differences observed between the parents and the crosses for dead heart and leaf feeding damage parameters is suggestive of the occurrence of exploitable heterosis for the development of genotypes that are resistant to stem borer attack. Genotypes SE NG-33, SE NG-65 and TZBR Syn W had high negative GCA values for dead heart while SE NG-62, SE NG-148, TZBR Syn W and TZBR ELD 3 C2 had the
high negative GCA values for leaf feeding damage. For ear damage, SE NG-65, SE NG-67, SE NG-119, SE NG-148 and AMA TZBR-W-C1 had high negative GCA estimates.
Genotypes SE NG-33, SE NG-62, SE NG-65, SE NG-77, SE NG-106 and SE NG-119 had the highest positive GCA effects for grain yield. The nine genotypes selected formed two heterotic pools: Group A comprised SE NG-33, SE NG-77, SE NG-106, SE NG-148 and TZBR Syn W while Group B included SE NG-62, SE NG-119, AMA TZBR-W-C1
and TZBR ELD 3 C2. Average yield of the grouped genotypes crossed in all possible
combinations was 1.06 t ha-1 showing 5 % yield increase. Furthermore, the best five yielding crosses namely; SE NG-33 x TZBR ELD 3 C2, SE NG-62 x SE NG-77, SE NG-
62 x SE NG-106, SE NG-106 x TZBR ELD 3 C2 and TZBR Syn W x TZBR ELD 3 C2,
selected may be used as population crosses or in the formation of composite varieties.
Maize (Zea mays L.) is the third most important cereal in the world after wheat and rice. In Nigeria, maize is popular and widely grown essentially because it matures during the “hunger period” and can be prepared in a variety of ways. In southern Nigeria, maize is a major component of the cropping system serving as hunger breaker while other crops are yet to mature.
In the rain forest zone of southern Nigeria, two crops of maize are possible per year due to the bi-modal rainfall pattern of the zone. The first season crop can be planted from mid March to first week of April while the second season planting is from mid August to early September. The maize produced in the early season is quickly consumed to avoid damage due to high humidity related diseases and pests. Storage is best with late maize during the onset of the dry season. Unfortunately, late season maize production is seriously limited by the activities of stem borers (Obi, 1991). The pink stem borer (S. calamistis (Hampson)) and the sugarcane stem borer (E. saccharina (Walker)) are the two stem borer species of economic importance in Southeastern Nigeria (Harris 1962; Appert, 1970; Bowden, 1976). The activities of the larva on the maize plants result in leaf feeding and stem tunneling, which in turn lead to reduced translocation of nutrients and assimilates, death of young plants (dead heart), lodging of older plants and direct damage to maize ears (Usua, 1968; Ezueh, 1978; Bosque-Perez and Mereck, 1990). All these damage activities tend to cause yield reduction and crop failure. Yield loss of between 10 to 100 % have been reported for stem borer attack in this region (Usua, 1968)
Control measures advocated for stem borers include direct use of insecticides, cultural control practices especially inter-cropping, early planting and good sanitation including burning of crop residue and the use of host plant resistance (HPR) (Lawani, 1982). Host plant resistance when strategically deployed in appropriate cropping system
is both cost effective and environmentally safe. Therefore, it is often regarded as the hub in any integrated pest management (IPM) intervention for stem borer control (Teetes, 1985; Kogan, 1982; Belloti, 1990).
Whenever good sources of resistance for desirable traits are identified, appropriate breeding methods, such as recurrent selection, can be employed to increase the frequency of such desirable genes in order to further increase productivity of such crop. Crop improvements depend mainly on the availability of genetic variability. Such variability can be obtained through introduction, selection from available variation, generated through mutation or through the use of biotechnological tools to obtain desired genes for desirable traits. Conventional method of developing resistant varieties involves the identification and use of resistant germplasm in breeding programmes. In looking for resistant sources, one approach is to search for germplasm in areas where stresses are prevalent. This approach can identify genotypes with resistance to local stresses including diseases and insect pests that are also adapted to local ecological problems such as low soil pH, low soil nutrient and root and stalk lodging (Fajemisin et al., 1985; Kim et al., 1985; Eberhart et al., 1991).
Maize is not native to Southern Nigeria therefore, all the maize varieties grown in this region must have been improved varieties introduced in not too distant past and maintained by the farmers over the years. Usually, farmers’ selections of seeds for the next crop represent a form of mass selection for tolerance to environmental stresses such as insect pests, plant diseases, drought etc. Evidence of exploitable genes for resistance to maize stem borers is available in literature (Ajala et al., 1995 and Ngwuta et al., 2001). At IITA, some sources of resistance to S. calamistis and E. saccharina have been identified and used to form TZBR populations (Bosque-Perez et al., 1989, Kling and Bosque-Perez, 1995). In the course of developing resistant populations, efforts were
aimed at breeding for resistance to these borers separately (Kling and Bosque-Perez, 1995). Some workers (Williams and Davis, 1984; Smith et al., 1989; Wiseman and Davis, 1990; and Mihm, 1995) have noted that the best strategy to a successful host plant resistance programme is the development of multiple insect resistant varieties. This approach is currently being used at IITA to develop genotypes with resistance to both Sesamia calamistis and Eldana saccharina (Schultess and Ajala, 1997; Ajala et al., 2002). The aim of this study was therefore to identify potential sources of multiple resistances to stem borers of interest and to generate genetically broad based reciprocal populations for further improvement efforts. Reciprocal populations have the advantages of complimenting each other for maximizing heterosis either as varietal crosses or in inbreds extracted from them and for continuous improvement of the two populations.
The objectives of this study were to:
i. evaluate local and a few improved populations from Southeastern Nigeria for agronomic traits and stem borer damage parameters
ii. investigate the major characters responsible for the variations among the maize genotypes assembled, and group them into homogenous subsets so that representative genotypes can be selected for further studies
iii. investigate the combining ability and heterosis for agronomic attributes and stem borer damage parameters in the selected genotypes, and
iv. identify the heteroic groups that can be used in inter-population improvement schemes for the development of high yielding varieties or hybrids.
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