Pre-empting Maize Lethal Necrosis Disease in South Africa: potyviruses of maize (Zea mays)

Abstract

The devastating Maize Lethal Necrosis Disease (MLND) hampers the production of maize, especially in developing countries. It causes mottling, necrosis and chlorosis of host plants, which results in severely reduced growth and yield and in extreme cases plant death in Zea mays (maize). Maize is regarded as one of three main staple food crops worldwide and in sub-Saharan Africa and South America it provides nutrition for over 1.2 billion people. Therefore, this disease is a severe food security threat and needs to be managed where it is present or pre-empted where it is predicted to spread to. MLND is caused by the co-infection of Maize chlorotic mottle virus (MCMV) and a cereal-infecting member of the Potyviridae family such as Sugarcane mosaic virus (SCMV), Johnsongrass mosaic virus (JGMV) or Maize dwarf mosaic virus (MDMV). MCMV is not present in South Africa, but was recently introduced into East Africa. The disease is predicted to disperse to neighbouring countries including South Africa (SA) in the near future. A number of potyviruses of maize have, however, been reported from SA, although the status, distribution and their diversity of these viruses has not been studied and a molecular tool to simultaneously detect these viruses has not yet been developed. In this study the status of potyviruses on maize in SA was determined, the diversity of the major maize potyviruses including those from SA and Tanzania was investigated, and an assay to simultaneously detect four maize-infecting viruses that are likely to be involved in MLND was developed. The results from this study allow us to pre-empt the introduction of MLND into SA by enabling us to 1) gain insight into which potyviruses are most likely to be present in potential MLND infections in SA and predict which areas are likely to be most affected, 2) better understand the evolution and diversity of the most common potyvirus on South African maize, SCMV isolates, in Africa and produce evidence that the MLN viral complex (or epidemiology thereof) will be more similar in SA to that seen in Asian countries rather than that seen in other African countries, and 3) verify that rapid, accurate and cost effective disease diagnosis can be made using the tool developed in this study. The conclusions of this study will contribute to future research regarding the viral components and epidemiology of MLND, especially in Africa. It also highlights some factors to consider in the development of genetically modified virus-resistant maize towards a durable control strategy to curb the impact of this disease. The maize industry, seed companies, quarantine services and research facilities working on MLN will benefit from the knowledge generated as well as the assay developed here. The study will also be useful in the subsequent selection of future research directions.