Combining ability analysis for resistance to turcicum leaf blight in maize under highland agro-ecologies of Uganda

A dissertation submitted to the Directorate of Research and Graduate Training in partial fulfilment of the requirements for the award of the degree of Master of Science in Plant Breeding and Seed Systems of Makerere University.

Maize productivity in the highland areas of Uganda is constrained by high prevalence of pests and diseases, and low levels of input use. Turcicum leaf blight (TLB) (caused by Exserohilum turcicum) is one of the most important constraints to maize production, hence the need to improve highland maize varieties for resistance to this disease. Gene action for resistance to diseases and the relationship between key morpho-agronomic traits are important considerations in crop improvement. This study was therefore conducted to (i) Determine combining ability of maize inbred lines for Turcicum leaf blight resistance, grain yield and selected agronomic traits in highland agro-ecologies of Uganda, (ii) Ascertain the genotype × environment interaction effects for Turcicum leaf blight resistance, grain yield and selected agronomic traits in maize genotypes under highland agro- ecologies of Uganda. (iii) Determine the correlations between Turcicum leaf blight response and other key agronomic traits of maize under highland agro-ecologies. A total of 18 single-cross hybrids generated from a North Carolina design II together with the nine parental lines and three checks were evaluated following an Alpha Lattice design with two replications in three highland agro-ecologies of Uganda namely: Bulegeni (1500 masl), Rwebitaba (1,523 masl) and Kalengyere (2,450 masl).Genetic analysis results indicated significant (P≤0.001) differences among test environments for grain yield, plant height, ear height, days to anthesis, and days to silking an indication that environments were distinct for the traits but non-significant for TLB. General combining ability (GCA) and specific combining ability (SCA) effects were significant (P ≤ 0.001) for most traits, indicating the importance of both additive and non-additive gene effects respectively. The SCA x Env interaction was not significant for all traits studied implying that the top yielding and best resistant hybrids were generally stable across environments. Inbred lines CKMARS10022, CKL14546, AMH704-43, AMH703-35, AMH704-51, AMH10142-29 and AMH 10142-10 had negative GCA effects for TLB resistance hence were selected as promising parents for TLB mitigation. The best performing genotype suitable for Kalengyere was identified as CKL14546 /CKDHL120671 as it had positive significant SCA for yield, negative SCA effects for TLB and positive effects for PH, EH, DA and DS. Hybrid CKMARS10022 /AMH703-35 though recorded a positive SCA for TLB it had positive significant SCA effects for yield, and negative SCA effects for DA and DS an indication that it is an early flowering variety hence best suited for Bulegeni region. Across the three locations, hybrids CKLTI0028/ AMH704-43, CKL14546 / AMH704-51, CKL14546 / CKDHL120671 and CKLTI0028 / AMH10142-29 were resistant while the rest were moderately resistant. The genotype variations observed was attributed to varied environmental factors and disease pressure. GGE biplot analysis revealed that hybrid CKL14546 / CKDHL120671 was stable across environments in relation to reaction to TLB disease. Parental genotypes and hybrids which possessed negative GCA and SCA can be recommended for breeding of TLB resistant cultivars. Considering the economic importance of TLB in highlands of Uganda, a large number of genotypes should be screened and the number of replications or hotspot environments also be increased during their evaluation. Molecular techniques can also be used to help understand the nature of the resistance governing the genotypes tested. Since there emerged presence of three mega-environments, it is fair to select Kalengyere as the screening site for the TLB due to its discriminative ability. More seasons and locations are recommended to completely reassert the magnitude of G x E and stability of the resistance.

MaRCCI, Makerere University