Distribution of maize lethal necrosis disease, its causal viruses and alternative hosts in the north - central regions of Tanzania

Maize is an important income generating food crop in Tanzania. However, yields remain low due to several limiting factors including among others diseases caused by fungi and viruses. The threat caused by several biotic factors in the country was further worsened with an outbreak of maize lethal necrosis (MLN) disease in 2012 in Arusha and Mwanza regions. Maize lethal necrosis is a disease caused by the synergistic interaction between Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV). Of these two, MCMV is a new virus in the African continent, first recognized in an MLN outbreak in Kenya, whereas SCMV is known to be endemic in Africa. This study focused on understanding the extent of MLN spread in north-central regions of Tanzania by conducting surveys between February and June, 2015. A total of 163 farmers‘ fields were sampled, in 14 districts in Arusha, Dodoma and Manyara regions. Incidence and severity were estimated based on MLN symptoms such as chlorosis, mottling and necrosis. The Enzyme – linked immunosorbent assays (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) methods were used for the detection of MCMV and SCMV in field samples. Disease severity assessed on a 1 to 5 rating scale varied from 2 to 3.6, with an overall mean incidence of 16.1%. Thirty nine percent of the samples tested positive to MCMV, 22% for SCMV and 5.5% for both MCMV and SCMV. A total of 254 non-maize crops and weeds tested for MCMV and SCMV, revealed SCMV in 7 samples (2.8%) in sugarcane (Saccharum officinarium), finger millet (Eleusine coracana), sorghum (Sorghum bicolor) and bristly foxtail ( bristly foxtail ( bristly foxtail (bristly foxtail (bristly foxtail (bristly foxtail ( bristly foxtail ( bristly foxtail (Setaria Setaria Setaria verticillataverticillataverticillata verticillataverticillata verticillata verticillata). The nucleotide sequence of the coat protein region of MCMV showed very high levels of homology (99%) between MCMV from Tanzania and those from Kenya and other countries. However, the SCMV nucleotide sequence of the coat protein region was divergent by up to 11%, compared to other isolates. This study confirmed the occurrence of MLN in 13/14 districts surveyed and also showed that SCMV occurs in maize as well as other cereal hosts. Further study of pathogen diversity and factors contributing towards disease occurrence is recommended as interventions for disease management are developed and deployed at various levels.