Epidemiology and Management of Maize Lethal Necrosis in Ethiopia

Abstract

Maize (Zea mays) is one of the most important staple food crops in the Eastern Africa region including Ethiopia. Maize lethal necrosis (MLN) is becoming a threat to maize production and has been challenging food security for the majority of households in East Africa since 2011. MLN is caused by a co-infection of maize chlorotic mottle virus (MCMV) with any one of cereal viruses in the genus Potyvirus, family Potyviridae, such as sugarcane mosaic virus (SCMV) maize dwarf mosaic virus (MDMV), wheat streak mosaic virus (WSMV) or johnson grass mosaic virus (JGMV). The general objective of this research was to study the epidemiology of MLN disease and its management through understanding the survival mechanisms of MLN-causing virures, insect-vector management and identification of resistance/tolerant maize genotypes. A field survey was conducted in major maize producing regions of Ethiopia from 2015-2018 to determine MLN geographical distribution, factors associated with disease intensity, alternate natural hosts, and type and distribution of insect vectors. Simple descriptive statistical analyses were performed to summarize the field survey data. The associations of MLN disease intensity with independent variables were analyzed using logistic regression. To describe and compare different categories of the sample units with respect to the desired characteristics, mean, standard deviation, and percentage were computed using simple descriptive statistics. This study revealed that MLN disease was distributed in major maize production areas of Ethiopia, especially in central, western, southern and southwestern parts of the country. The disease was most prevalent in Southern Nation, Nationality and Peoples (SNNP) region with 66.67% prevalence followed by Oromia region that had a prevalence of 65.62%. Natural MLN alternate host assessment uneder field condition and host range study by artificially inoculating in the greenhouse were conducted to determine potential alternate hosts. Poaceae family had the highest number of grass species that were alternate hosts for MLN causing viruses. Digitaria sanguinalis, Phalaris paradoxa, Oplismenus hirtellus, Echinocloa colona, Cynodon nlemfuensis, Pennisetum purpureum from Poaceae and Cyperus cyperoids from Cyperaceae family were naturally infected by MCMV. Cyperus rotundus, sugarcane (Saccharum officinarum) and sorghum (Sorghum bicolor) were infected by both MCMV and SCMV under natural field conditions. In addition, seed transmission study was conducted using growing-on tests to determine the potential of seed transmission and its role in the spread of MLN. The mean overall seed to the seedling transmission rate of MCMV was 0.073% with a range of 0 to 0.17% among 20 different maize genotypes studied. Out of 20 maize genotypes evaluated, 14 genotypes had some levels of seed transmission (0.03-0.017%) for MCMV whereas SCMV seed transmission was observed only in a single plant of one genotype, with an overall average of only 0.003%. The rates of seed transmission of the viruses were influenced by the seed lot and maize varieties used. Field assessment, laboratory, and greenhouse experiments were also conducted using MCMV and SCMV infected maize residue and infested soil to assess the role of MLN infected maize residue and infested soil in the transmission of MLN causing viruses. Serological detection and back-inoculation test result showed that MCMV was detected and confirmed to be transmitted from infested soil to newly raised maize seedlings. However, SCMV was neither detected in soil samples from infected fields nor transmitted to maize seedlings. Under the experimental condition, MCMV remains persistent and transmissible up to 6 months in maize planted on MLN infested soil mixed with MLN infected maize residue. Registered systemic seed dressing insecticides in Ethiopia were evaluated either singly or in combination with fungicide against identified insect vectors of MLN causing viruses. Among seed dressing insecticides evaluated against Franklinella sp and R. maids, thiamethoxam 25% at 2.0g/kg seed and imidalm T 450 at rate 1.5 were showed the superior control efficacy. Vectors were introduced on to the maize seedlings developed from seeds treated with insecticides and the percentage of the reduction of the insect population was determined. Based on morphological and PCR-based vector’s identification, transmission and subsequent mechanical inoculation tests, maize thrips (Franklinella sp.) and cereal leaf beetle (Oulema sp.) were identified as potential vectors of MCMV, while corn leaf aphids (Rhopalosiphummaidis) was a potential vector of SCMV. A greenhouse screening study was conducted by artificially inoculating MLN-causing viruses to determine the reactions of various maize genotypes to MLN to identify resistant genotypes that can be either utilized in breeding programs or recommended for commercial production. Nearly 7%, 16.7% and 5.5% of the inbred lines from highland, mid-altitude and lowland maize breeding programs, respectively, showed moderately resistant reaction to MLN. Higher proportion of the inbred lines and varieties showed susceptible to highly susceptible responses. The various weed and cultivated plants identified as alternate hosts, the insect vectors, the transmissibility from infected seed and infested soil to newly raised maize seedlings, and the persistence in soil and maize residue of MLN causing viruses, are believed to be epidemiologically important and maintain the virus inoculum in the absence of maize crop in the field. These also support the survival of the virus for continuous infection. As part of an integrated management of MLN, farmers and stakeholders involved in maize production should take precautionary measures by using certified and virus-free maize seeds from trusted sources; regular field monitoring, assessment of virus symptoms and rouging-out diseased maize plants; good field sanitation methods including weed control and eliminating alternate host plants within and in the surrounding areas of maize fields together with seed dressing before planting with thiamethoxam 25% WG @ 2.0 g/ kg seed or Imidalm T 450@1.5 g/kg of seeds for early-stage protection.