Genetic value of maize populations for resistance to diseases

Author(s)

Dario Rosa Mesquita Neto

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2000-04-05
• Advisor: Jose Branco de Miranda Filho

Abstract

Maize breeding programs in general concentrate efforts on adapted and improved germplasm, invariably resulting in a relatively narrow genetic base, exposing the currently used cultivars to vulnerability to old or new races of pests and diseases. The germplasm banks keep collections of accesses that can contribute to new sources of disease resistance in maize. However, all over the world new sources of germplasm have not been extensively used because they do not fulfill the market requirements in terms of acceptable agronomic traits. Under this circumstance the pre-breeding strategy tums to be a feasible alternative to prepare non conventional genetic materials for use in breeding programs. Looking for an optimization of the germplasm use in breeding programs, this work followed the objectives: i) to evaluate and study the genetic value of maize accesses relative to the performance per se and in crosses with testers; ii) selection of the outstanding accesses, based on general combining ability, toward the synthesis of composites with acceptable pattern for agronomic traits and high concentration of alleles for disease resistance. In the agricultural year of 1994/1995, the NAP-MILHO (Núcleo de Apoio à Pesquisa em Milho) Project was organized in a cooperative program involving public and private institutions. The project initiated with the evaluation of 1273 accesses from Germplasm Bank (CNPMS/EMBRAPA) in 13 locations over five states in Brazil. Traits evaluated comprised grain yield, resistance to com stunt and leaf diseases including those caused by Exserohilum turcicum or northern leaf blight, Phaeosphaeria maydis or Phaeosphaeria Ieaf spot, Physopella zeae or tropical rost and Puccinia polysora or polysora rust. The best accesses for yield and disease resistance were selected and divided into two sets: Set I with 28 early accesses; and Set II with 29 extra-early accesses. The two sets of accesses were evaluated in two different mating schemes: A) intergroups partial diallel cross where the group of accesses were crossed with a group formed by F2 generations of commercial hybrids; and B) intragroup top-cross, where the accesses were crossed the mixture of the whole set of accesses as tester in each set. Experiments followed the complete randomized block design in three locations and in two seasons (normal and off-season or "safrinha"). All the diseases previously considered in the NAP-Project were evaluated in the experiments, including also the reaction to Puccinia sorghi and stalk rot, stalk lodging, stalk breaking, grain yield and other traits. For some diseases, there were the following types of reaction: resistant, moderately resistant and moderately susceptible. Common rust, com stunt and stalk rot were the main diseases of the "safrinha" planting. The tropical and polysora rusts occurred only in the normal season at a moderate level of attack. The Phaeosphaeria leaf spot was the only disease that occurred in all the experiments in the two planting seasons, and was the main disease in the normal season. Accesses with acceptable patterns for disease resistance and yield were found. In general, the extra-early accesses showed higher levels of severity for many diseases and also showed to be more susceptible to lodging and stalk breaking, but they were similar to the set of early accesses for yield. ln the normal season, the grain yield was higher in the area of Caterpillar (4,329 kg/plot) as compared to Rio Verde (3,276 kg/plot), where yield was higher than in Anhembi (2,799 kg/plot). In Rio Verde grain yield was higher in the normal planting season as compared to the off-season planting or "safrinha" (2,198 kg/plot). However, in the absence of diseases in the "safrinha" planting some accesses and many crosses showed to be higher yielding than the less yielding check. It was found significant average heterosis (h̄) for the following traits: reaction to the Phaeosphaeria leaf spot and the corn stunt, percentage of lodging, plant height, ear height and grain yield. Except for the corn stunt, that practically there was not incidence in the diallels, for the other traits larger estimates of average heterosis were obtained in the cresses with the F2 generations of commercial hybrids, indicating that there was genetic divergence between the parent groups and that a better performance was observed in crosses between the accesses and commercial hybrids, as compared to crosses among the accesses themselves. Depending on the parents involved in the crosses, the variety effects (vi) were more important in some cases, and the effects of variety heterosis (hi) in others. In both statistical-genetic models, independent of the level of significance for the variation among parents (accesses and hybrids) or for the variety heterosis effects, the effects of general combining ability (gi) were estimated and used for the selection of the most promising parents to be intercrossed for the synthesis of new composites to be used as base populations in recurrent selection programs. (AU)