SUMMARY Maize streak virus (MSV; Genus Mastrevirus, Family Geminiviridae) occurs throughout Africa, where it causes what is probably the most serious viral crop disease on the continent. It is obligately transmitted by as many as six leafhopper species in the Genus Cicadulina, but mainly by C. mbila Naude and C. storeyi. In addition to maize, it can infect over 80 other species in the Family Poaceae. Whereas 11 strains of MSV are currently known, only the MSV-A strain is known to cause economically significant streak disease in maize. Severe maize streak disease (MSD) manifests as pronounced, continuous parallel chlorotic streaks on leaves, with severe stunting of the affected plant and, usuallly, a failure to produce complete cobs or seed. Natural resistance to MSV in maize, and/or maize infections caused by non-maize-adapted MSV strains, can result in narrow, interrupted streaks and no obvious yield losses. MSV epidemiology is primarily governed by environmental influences on its vector species, resulting in erratic epidemics every 3–10 years. Even in epidemic years, disease incidences can vary from a few infected plants per field, with little associated yield loss, to 100% infection rates and complete yield loss. mpp_568 1..12

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Maize streak virus: an old and complex 'emerging' pathogen.

Eight decades of maize streak virus research

Maize streak virus (MSV), the causal agent of maize streak disease (MSD), is one of the most significant biological threats to food security in sub-Saharan Africa. By reducing yields of Africa’s most important food crop, it seriously undermines the already precarious social and economic wellbeing of subsistence farmers throughout the continent. Despite the availability of MSD control strategies—ranging from good farm management to the use of chemical insecticides, and the planting of MSD tolerant maize –MSD remains a problem. The main reason for this is the inherent unpredictability of MSD epidemics which makes it difficult for farmers to decide where and when to apply appropriate control strategies. Also, without better estimates of the economic impacts of the disease, there is unlikely to be any motivation for large scale efforts by scientists and national governments to refine and apply effective control measures. Besides reviewing the epidemiological factors that would need to be modeled to achieve predictive forecasting of epidemics, we approximate how much MSD costs national economies in sub-Saharan Africa and evaluate various possible approaches to minimizing the over-all economic impact of the disease.

The epidemiology, economic impact and control of maize streak disease

The distribution of leafhopper vector populations and the viral diseases they vector are inherently influenced by agro-ecological factors. This review discusses the distribution and classification of MSV strains, their vectors and how agro-ecological factors mediate the prevalence of MSV disease. Important strains of MSV have been characterised on the African continent using molecular techniques, however, evidence on the characterisation of geographically separate populations of Cicadulina spp. and the viral strains they vector is inadequate. The potential of applying knowledge on the influence of soil nutrients, altitude and temperature on the biology of maize streak virus (MSV) / vector populations is discussed. This review has considered the potential of using soil nutrients in determining nutritive status of maize host plants and its effects on population dynamics of Cicadulina mbila Naude (Homoptera: Cicadelidae), an important vector of MSV disease and the expression of MSV disease. Future research options that can provide information on the influence of soil fertilization on the behaviour of characterised vector populations, MSV disease transmission, virulence and disease expression are proposed.

https://www.ajol.info/index.php/ajb/article/download/59697/47977

Significance and transmission of maize streak virus disease in Africa and options for management: A review

The phylogeography, epidemiology and determinants of Maize streak virus dispersal across Africa and the adjacent Indian Ocean Islands

Regular field surveys were carried out in 2 week intervals in 1986 and 1987 to determine population density, species composition, migration activity of Cicadulina leafhoppers and their infectivity with maize streak virus (MSV) in different ecological locations of southeastern Nigeria: mangrove swamp forest (IITA and RIART station at Onne), deltaic swamp forest (RSUST, Port Harcourt), seasonal swamp forest (Iriebe) and high forest zone (NCRI, Amakama near Umuahia). Population density was always low during dry season, significantly lower in the rain forest zone and higher in the swamp forest, rapidly rising in the second rainy season in all ecological zones and reaching its maximum before the rains finally stopped. Sharp increase in the population density in Nov. and Dec. may indicate mass migration activity of Cicadulina leafhoppers from already dry areas of adjacent Guinea savanna at that period. Six species of Cicadulina were collected from most of sampled areas. C. ghaurii was the predominant species in 1986 and 1987, averaging 55.9% for all locations and periods. Its portion was higher in the mangrove swamp forest (Onne) than in the high forest zone (Amakama). The predominance of other species declined in the order: C. triangula (18.2%), C. mbila (17.2%), C. arachidis (5.9%), C. similis (2.8%) and C. hartmansi (0.1%). In absolute values, the portion of individuals out of the total number of leafhoppers collected was for C. triangula 53.7%, C. ghaurii 25.5%, C. mbila 17.3%, C. arachidis 2.3%, C. similis 1.1% and C. hartmansi 0.1%. A high population of C. triangula occurred only in the relatively short period in Nov.–Dee., especially in Iriebe and was correlated with the rapid increase of Cicadulina densities in all sampled areas; a high migration activity of C. triangula and C. mbila was confirmed. Incidence of MSV in the surveyed farmers’ field varied between 0–22% in the first rainy season (1987) and 12–25% in the second rainy season (1986). The portion of Cicadulina adults already infected with MSV under field conditions and capable of transmitting MSV to the susceptible maize seedlings varied from 3.3 to 23.3%, with higher values for leafhoppers collected at the end of the second rainy season.

F. Nwilene

Papers

First regular observations on leafhoppers, Cicadulina spp., vectors of maize streak virus (MSV) in Southeastern Nigeria