Distribution and Characterization of Rice yellow mottle virus: A Threat to African Farmers.

Abstract

Africa produces only 2.7% of the world’s rice and is the second-largest riceimporting region in the world (6.5 Mt in 2003). This amount represents about 25% of the world rice importation (40). With an average of 2 t/ha, excluding Egypt, rice production in Africa remains significantly below the world average (Asia 3.8 t/ha, Latin America 3.0 t/ha, United States 7.0 t/ha; Food and Agriculture Organization of the United Nations [FAO], published online). Insects and diseases are the two major constraints limiting rice production in Africa and Asia. Of all the rice diseases, the one caused by the Rice yellow mottle virus (RYMV), discovered in Kenya in 1970, is one of the most damaging in Africa. RYMV has been reported in many countries in East and West Africa, where in some cases whole fields have been devastated. RYMV has only been reported from the African continent and is endemic in every country where it has been reported. RYMV is a member of the genus Sobemovirus and possesses all the characteristic biophysical and biological properties of the members of the genus. The major insect vectors are chrysomelids, which play an essential role in primary infection, while secondary infection is due mainly to plant-to-plant contact. The virus particles are stable in infected dried leaves. Limitations to the spread of the virus include restricted mobility of the insect vectors, a limited host range, and absence of seed transmission. Partially resistant and highly resistant varieties have been identified, but currently the disease is not controlled adequately, and its incidence is increasing significantly in Africa. Due to the importance of rice as a staple food, the scientific community is active in studying the virus and the resistant mechanisms in rice. At the Institut de Recherche pour le Developpement (IRD) and the International Laboratory for Tropical Agricultural Biotechnology (ILTAB), different tools, such as RYMV polyclonal and monoclonal antibodies, have been produced for RYMV detection; and a series of studies has been initiated for a better understanding of the virus–host interactions. The synthesis of an infectious clone, the sequencing of many isolates of the virus, and structural studies of virus particles permitted a study of the function of the different viral proteins and an increased understanding of the virus cycle. African virologists also have initiated studies on the epidemiology of the RYMV, mainly the virulence and variability of virus strains collected throughout Africa (36,45). The relationships between genetic variability of the virus and geographical distances have been studied (19). Currently, phytopathologists are working in the field to better assess the agronomic impact of the virus (36,61,62). Breeders and geneticists at IRD and West African Rice Development Association (WARDA) in Cote d’Ivoire are working together to identify and isolate natural resistance genes (6,41). The International Institute for Tropical Agriculture (IITA) in Nigeria undertook breeding programs in Africa. Molecular biologists are offering new ways of controlling the disease through production of transgenic rice plants (37,52). This article is intended to provide a review of the disease and give an update on recent research results. The disease’s impact on rice production in Africa, its distribution, symptomatology, and epidemiology, the physical and genetic characteristics of the virus, and resistance mechanisms and breeding efforts for resistance will be discussed.