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Breeding investigations for developing durable resistance to maize lethal necrosis disease (MLND) and its causal viruses in Kenya.

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Maize is an important staple food in Kenya, grown in almost all agro-ecological zones, and accounts for about 40% of daily calories. The country produces an average of 3.51 million tons of maize grain (39 million x 90 kg-bags) which does not match the demand. The production is limited by emerging pests and diseases, such as fall army worm (FAW) and maize lethal necrosis (MLN) disease. The MLN disease is caused by synergisim between maize chlorotic mottle virus (MCMV) and sugar cane mosaic virus (SCMV). The FAW and MLN cause serious challenges for maize production due to the lack of suitable cultural control methods and tolerant maize germplasm. As a result, farmers and government agencies have challenged maize breeders to develop varieties that are resilient to climage change. This study focused on breeding investigations for developing durable resistance to maize lethal necrosis disease and its causal viruses in Kenya. The specific objectives were to 1) identify maize germplasm lines that are tolerant to MLN and its causative viruses (SCMV and MCMV) for use in making hybrid combinations; 2) to understand the genetic divergence and background of these genotypes to aid in developing high yielding and stable MLN resistant maize hybrids; and 3) to assess the impact of MLN, SCMV, and MCMV on maize production in Kenya. Experiments were conducted in Kenya during the 2015, 2016, 2017 and 2018 seasons in both screen house and open fields. Diallel crosses were used to characterize maize inbred lines, from CIMMYT, KALRO, and Ohio, for reactions to SCMV, MCMV and MLN disease. A set of selected and pre-commercial maize hybrids were used to assess MLN impact on maize production in Kenya. The study revealed that MLN is important for maize production, with high incidence and severity levels observed in all commercial maize varieties in Kenya. Disease severity, yield and its attributes significantly varied (P < 0.05) at V3, V7 and VT inoculation stages. The greatest effect was observed at V3 stage with increased number of rotten cobs observed at VT. Percentage yield loss was proportional to the percentage of disease incidence, severity and effect on yield attributes which varied from variety and season. This calls for the development and deployment of improved maize hybrids with MLN tolerance/resistance in the MLN-prone areas to enhance maize production. The study further identified eight inbred lines with low disease severity < 3.0 at 56dpi indicating their tolerance/resistane to SCMV, MCMV and MLN. The maize inbred lines MLN001 and MLN006, displayed high levels of resistance to MCMV, while MLN042 and MLN041 had the highest resistance to SCMV. Strong sources of MLN resistance, such as MLN013, MLN019, N211, and KS23-6, should be used in developing MLN-resistant hybrids. Inbred lines CML312, CML442, MLN013, N211 and MLN 019 are identified as good combiners for yield and earliness. The nature of gene effects was established as one including additive, and that genotype x location effects were significant for conditioning MLN resistance in maize hybrids. The presence and impact of SCMV and MCMV viruses in the field need to be investigated to aid in developing more robust MLN resistant varieties. MNL001, MLN013, MLN018, and MLN019 and their derived crosses need to be evaluated across maize agro-ecologies and seasons to understand MLN disease, the role of environment and the interaction of MCMV and Potyviruses.


Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.