Maize (Zea mays L.) is the third most important cereal crop after wheat and rice (FAO, 2011). During 2006 the world maize production was 144 million hectares while that of wheat was 216 million and rice 154 million hectares (FAO, 2008).The crop occupies a pivotal role in the world economy and is traded widely. Maize demand is projected to increase by 50% worldwide and by 93% in sub-Saharan Africa between 1995 and 2020 (Rosegrant et al., 2001; FAO, 2007). In the past, much of the global use of maize has been for animal feed. However, maize is increasingly used for human consumption and accounts for 70% of the food consumed in sub-Saharan Africa (FAO, 2007). Because of its productivity and wide adaptation, maize remains an important source of food with great potential to improve the livelihoods of most poor farmers in developing countries (FAO, 2011).
Maize productivity is limited due to a number of biotic and abiotic stresses. The major abiotic stresses affecting maize production included drought and soil nutrient deficiency and the biotic stresses are: infectious diseases and pests such as maize stem borer, weevils and termites (Mosisa et al., 2012; Girma et al., 2012). The major diseases of maize include Turcicum leaf blight caused by Exserohilum turcicum Pass Leonard &
Suggs, grey leaf spot (Cercospora zeae-maydis Tehon & Daniels) and common leaf rust (Puccinia sorghi Schr.) (Tewabech et al., 2012). Among the maize diseases, Turcicum leaf blight also known as northern corn leaf blight (NCLB) is a wide spread disease with incidence ranging from 95 – 100% in areas with constant moisture and high humidity occurring in the main rainy season (Juliana et al., 2005; Tewabech et al., 2012). Also, the disease causes qualitative changes in the seed resulting to decreased sugar content, germination capacity and heavily infected plants are predisposed to stalk rot (Bowen and Pedersen, 1988; Cardwell et al., 1997; Muiru et al., 2007). The disease is ranked as the number one problem and is considered a high research priority of maize in Ethiopia (Wende et al., 2013).
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The NCLB of maize is one of the widely distributed and economically important diseases of maize in sub-Saharan Africa (Tewabech et al., 2012; Wende et al., 2013).
Infection of the disease appears during both off and main production seasons. However, disease epidemics are more pronounced during the main-season especially in constantly wet and humid areas. Therefore, strategic breeding to develop resistant varieties is crucial in areas where the disease reaches epidemic proportions (Girma et al., 2008).
Different options are available to control maize leaf blight such as the use of host plant resistance, cultural practices, and fungicides (Girma et al., 2008; Meseret and Temam, 2008). Host plant resistance is the cheapest and most effective way to control leaf blight disease because chemical treatments are expensive, often ineffective, and sanitation practices are difficult to apply. The use of resistant varieties possessing qualitative and quantitative genes in combination or separately is the cheapest and environmentally friendly method (Juliana et al., 2005; Dagne et al., 2008). Two forms of host plant resistance are distinguishable: qualitative and quantitative. Qualitative resistance is race-specific and governed by a single or few gene(s) whereas quantitative resistance is race-non-specific and polygenic (Singh et al., 2004; Ogiliari et al., 2005). Qualitative resistance genes such as Ht1, Ht2, Ht3, Htn and Htm are reportedly dominant or partially dominant and confer nondurable resistance. This form of resistance may break down due to emergence of virulent races of the pathogen through genetic mutation and recombination events (Freymark et al., 1994; Weilz and Geiger, 2000; Juliana et al., 2005; Ogiliari et al., 2005). E. turcicum exhibits a wide range of variability (Yeshitela, 2003) and new races overcoming previously resistant varieties are documented (Juliana et al., 2005). Most breeding programs rely on qualitative resistance conferred by Ht genes.
Resistance conferred by the Ht gene(s) are characterized by chlorotic and necrotic lesions or lesions surrounded by a yellow to light brown margin, without spore formation, which limits the growth and spread of the disease (Jungenheimer, 1976;
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Hooker et al., 1977; Weilz and Geiger, 2000; Singh et al., 2004). Typically, resistance conferred by the Htn gene is expressed as a delay in lesion formation and presence of fewer lesions (Gevers, 1975; Ogiliari et al., 2005). Varieties with the Htn gene generally remain free from lesion development until pollination (Leonard, 1989). Polygenic resistance conferred by minor genes is not absolute when compared to qualitative or monogenic or oligogenic resistance. However, minor gene resistance is more durable and chances of new pathological races breaking the resistance are relatively minimal (Ojulong et al., 1995; 1996). Polygenic resistance effectively reduces the rate of disease increase (Vanderplank, 1963; Parlevliet, 1979). Genotypes with this resistance can vary from highly resistant showing a few lesions to more susceptible reaction types where large sporulating lesions are present (Elliott and Jenkins, 1946; Meyer et al., 1991).
Breeding for resistance or tolerance to E. turcicum is the most economically viable option to release varieties for resource-constrained farmers. This is achieved through incorporation of resistance genes into the existing elite genotypes. The option serves as one of the major components in the integrated management of the maize leaf blight.
Disease severity, disease incidence, lesion size, and area under disease progress curve are the most common parameters used in the evaluation of maize genotypes for resistance to Turcicum leaf blight (Adipala, 1994; Pratt et al., 2003).
Due to the economic importance of Turcicum leaf blight disease various national and international programs are actively involved in breeding for resistance. However, some of the commercial varieties as well as elite parental inbred lines are reportedly vulnerable to Turcicum leaf blight (Njuguna et al., 1990; Muthinda 1997; Welz and Geiger, 2000). There is a continued need to identify new sources of resistance through artificial inoculation or natural epidemics of the disease among available breeder’s genetic stocks and introduced germplasm for breeding, disease management and to enhance maize productivity (Girma et al., 2008).
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In Ethiopia maize productivity is low, with the estimated national average yield of 2.5 t ha-1 due to several abiotic and biotic constraints (CSA, 2010). Bako national maize research project in Ethiopia is involved in maize research and development. The project, situated in the region of mid-altitude, sub-humid maize growing agro-ecology, aims to enhance maize productivity through effective breeding using locally adapted germplasm as well as through well-designed hybrid cultivar development. Recently the project embarked on a dedicated resistance breeding program to develop leaf blight resistant varieties through incorporation of resistant genes into well-adapted but susceptible germplasm for sustainable production across the mid-altitude sub-humid agro-ecologies. Therefore, the objective of this study was to determine the genetic variability among 50 elite maize inbred lines and select promising lines with leaf blight resistance and adaptation to the mid-altitude sub-humid agro-ecologies. The selected lines may be used in resistance breeding programs to minimize losses incurred by Turcicum leaf blight in maize.