Chapter 1. Maize lethal necrosis (MLN) in Africa: incidence, impact, rapid response, and management

Maize (Zea mays L.) is the most important cereal crop in sub-Saharan Africa (SSA), covering over 40 million ha, largely in smallholder farming systems, with a production of over 70 million metric tonnes (MMT) of grain (FAOSTAT, 2021). The crop is critical for food security, incomes, and livelihoods of several million smallholders across SSA, especially in eastern and southern Africa where nearly 85% of the maize produced is used as food (Shiferaw et al., 2011). However, average maize yield in SSA (~2 t/ha) is far below the global average (~5 t/ha), due to various reasons, including frequent occurrence of drought, poor soil fertility, inadequate use of inputs (both improved seed and fertilizers), and challenges imposed by various pests and diseases (Prasanna et al., 2021). The spread of transboundary pests and diseases has increased significantly in the recent years, affecting the food security and livelihoods of several million resource-constrained smallholders, especially in SSA, Asia, and Latin America. Globalization, trade, and climate change, as well as reduced resilience in production systems due to decades of agricultural intensification, have all played a part. One such major example is the emergence of maize lethal necrosis (MLN) in sub-Saharan Africa, which was first reported in the southern Rift valley area of Kenya in 2011 (Wangai et al., 2012), and then rapidly spread to several other eastern Africa countries during 2012 to 2014 (Mahuku et al., 2015; Redinbaugh and Stewart, 2019; Prasanna et al., 2020). MLN is a viral disease caused by combined infection of maize plants with Maize chlorotic mottle virus (MCMV) with any one of the members of family Potyviridae, such as sugarcane mosaic virus (SCMV), Maize dwarf mosaic virus (MDMV) or Wheat streak mosaic virus (WSMV) or Johnson grass mosaic virus (Stewart et al., 2017). MCMV was a recent introduction into eastern Africa, possibly in 2011, while SCMV has a worldwide distribution, including in SSA, over many decades. Therefore, the outbreak of MLN in Africa was primarily triggered by the introduction of MCMV.