Melhoramento genético da resistência à helmintosporiose comum, cercosporiose e ferrugem-polissora em milho-pipoca

Abstract

The development of disease resistant genotypes are a major goals of corn breeding. In this attempting, the identification of promising parental genotypes and to the estimation of genetic parameters and genes effects are required to direct the procedures towards higher gains from selection. The goals of this work were: (i) to investigate the possibilities of breeding, identifying inbred lines and hybrids which are resistant to northern leaf blight (Exserohilum turcicum), gray leaf spot (Cercospora spp.) and southern rust (Puccinia polysora) resistances; as well as to study gene effects and genetic parameters to decide about breeding strategies; (ii) to propose a new diagrammatic scale to assess northern leaf blight, providing reliable assessments to determine resistance levels; (iii) to evaluate the reaction of new popcorn hybrids to northern leaf blight and gray leaf spot, indicating the best ones to advancement in the program. To achieve the goals (i) e (iii), test under field conditions were conducted and diseases severity assessment were carried out. Data sets were examined by parametric analysis of variance, diallel analysis and non-parametric tests. To the goal (ii), leaves were collected and actual severity was measured. Then, severity on leaves was estimated using the new and other scales and data set was submitted to linear regression analysis. General and specific combining abilities were significant (p < 0.10) to northern leaf blight and gray leaf spot resistances. General combining ability was predominant, indicating that additive genes were more important and intrapopulation methods of recurrent selection can be effective. Inbred lines P8 and P9 were the most contributed to northern leaf blight resistance. Inbreds P3 and P4.3 were the most indicated to gray leaf spot resistance. In this context, it was recommended the composite 2 × 4 × 8. The estimates of genetic parameters indicated more difficulties to breed towards gray leaf spot resistance than to northern leaf blight. To southern rust, it was verified significant differences (p < 0.10) to group 2-Zaeli general combining ability, in which the inbreds P8.5 and P8.2 were the most contributed to resistance, being indicated to segregant populations to selection. General combining effects predominated over those of specific ability, indicating that additive genes are more important to southern rust resistance and suggesting that intrapopulation methods of recurrent selection might be more effective. The new scale increased precision, accurate and reproducibility of northern leaf blight severity estimates. Then, this scale was considered the most efficient methodology to obtain reliable data sets and compare genotypes in resistance. The 42 hybrids were classified as resistant to northern leaf blight and eight of them were medium susceptible to gray leaf spot. Using advanced values, 26 new hybrids might be advanced to subsequent tests.