Diallel analysis of maize resistance to Phaeosphaeria maydis

Abstract

In breeding programs directed towards genetic resistance against diseases, the estimation of genetic parameters that control resistance allows the introduction of resistance into suscetible germplasm to be clearly focused. The purposes of this study were to estimate heterosis effects, and the general (GCA) and specific (SCA) combining abilities by using two rating methods for resistance to Phaeosphaeria maydis in a diallel analysis of 36 F 1 maize hybrids and their nine inbred lines. Trials were conducted in three environments. Disease severity was evaluated in the whole plant (PI) and in the leaf positioned just below the point of insertion of the main ear (AFA). The trials followed a randomized block design with plots represented by a 5 m long rows. Differences among combining ability estimates for different environments and in both evaluation methods showed significant effects (P < 0.01) for environment (E), GCA, and GCA × E. The SCA, and SCA × E effects were not significant for any of the disease severity variables. The GCA effects were more important than SCA for this set of inbred lines, suggesting that additive genetic effects are the most important sources of variation for this trait. Heterosis effects for resistance were estimated, and it was possible to identify specific hybrid combinations between lines which have high potential for genetic control of this pathogen. Results for both disease severity variables were practically identical, even though the PI method was more convenient to use. Key words: Zea mays L., genetic resistance, heterotic effects, general and specific combining ability ANÁLISE DIALÉLICA DA RESISTÊNCIA A Phaeosphaeria maydis EM MILHO RESUMO: Em programas de melhoramento visando resistência genética a doenças, a estimativa de parâmetros genéticos que governam a resistência permite direcionar a introdução de resistência em germoplasmas. O objetivo deste trabalho foi estimar os efeitos heteróticos, a capacidade geral (CGC) e específica (CEC) de combinação, utilizando-se de dois métodos de avaliação da resistência, à Phaeosphaeria maydis através da análise dialélica de 36 híbridos F 1 e de suas nove linhagens genitoras, em experimentos conduzidos em três ambientes. Foi utilizado um delineamento experimental em blocos casualizados com três repetições e a parcela experimental foi representada por uma fileira de 5 m. As diferenças entre as estimativas da capacidade de combinação, em diferentes ambientes e para os dois métodos de avaliação, apresentaram efeitos significativos (P < 0.01) para ambientes (E), CGC e CGC x E. O efeito de CEC e a interação CEC x E não foi significativa para os dois métodos de avaliação. Os efeitos de CGC foram mais importantes que CEC nesse conjunto de linhagens, sugerindo que efeitos genéticos aditivos são mais importantes como fonte de variação para resistência a esta doença. Efeitos heteróticos para resistência foram estimados, sendo possível identificar combinações híbridas específicas entre linhagens com alto potencial para o controle genético des...

Diallel analysis of grain yield and resistance to seven diseases of 12 African maize (Zea mays L.) inbred lines

“…In other studies, GCA accounted for 100% of variation for GLS resistance (Thompson et al 1987;Ulrich et al 1990). Studies on the mode of inheritance of PLS indicate preponderance of additive effects and a smaller role of dominance action in conferring resistance in American and Brazilian materials (Carson 2001;Silva and Moro 2004). Resistance in maize to MSV is controlled by a major gene (explaining 50-60% of phenotypic variation), with two, three or ''few'' modifiers (Efron et al 1989;Kim et al 1989;Pernet et al 1999a, b).…”

Gene action determining Phaeosphaeria leaf spot disease resistance in experimental maize hybrids

“…Percent leaf area diseased (%LAD) also ranged significantly between 0.3% for the most resistant hybrids, and 45-50% for the most susceptible hybrids ZS255, SC513 and B24/B18 (data not shown), which is comparable to findings in Brazil. Pegoraro Silva & Moro, 2004). This late arrival in the crop cycle increases the chances that the disease severity could remain low and cause little yield loss (Carson, 2005b).…”

“…This late arrival in the crop cycle increases the chances that the disease severity could remain low and cause little yield loss (Carson, 2005b). Under favourable conditions, the onset and development of PLS may be relatively early (occurring at the vegetative stage), such that severe necrosis occurs, reducing photosynthetic area and grain filling (da Silva & Moro, 2004). The frequency distribution of the PLS scores in the 80 maize hybrids was skewed towards resistance.…”

“…Over the past six years, PLS incidence has.been increasing in South Africa, Zimbabwe, Kenya and Cameroon. It is mainly prevalent in high rainfall, cool and high altitude areas (Carson, 2001;Silva & Moro, 2004). Although yield loss assessments have not been conducted in southern Africa, substantial yield losses have been reported in Brazil.…”

“…These inbred lines represent the two major heterotic groups used in USA hybrid programmes. Additive gene action has been found to be predominant, while dominant gene action has been reported to playa very minor role in controlling PLS resistance in American and Brazilian germplasm (Carson, 2001;Silva & Moro, 2004). Carson (2001) reported that incomplete dominance was conferred by about four genes in Mo17; whereas Pegoraro et al (2002) reported two major independent genes that controlled PLS resistance in an additive manner in Brazilian maize.…”