Breeding rice for rice yellow mottle virus resistance in Sub-Saharan Africa: a review

Rice is the world's second most produced staple cereal crop after wheat. Currently, rice production and consumption have steadily increased in Sub-Saharan Africa (SSA). To date, rice is the largest imported commodity crop in the region. The low productivity is due to a number of biotic and abiotic stresses, and socio-economic constraints. Among the biotic constraints, rice yellow mottle virus (RYMV) is the most important constraint to rice in SSA. In SSA, RYMV causes yield losses ranging from 20% to 100%. Various control strategies (host resistance, cultural practices and chemicals) have been recommended to manage RYMV epidemics. RYMV disease management through generic crop protection chemicals is not economic nor is it successful due to the presence of a large number of vector species disseminating the virus. In addition, cultural practices are ineffective against RYMV because the virus is spread by several agents, including insect vectors. The use of RYMV resistant cultivars remains the most effective, economic and environmentally friendly method for resource poor farmers. However, RYMV resistant varieties have not yet been developed and deployed in SSA. The aim of this review was to present the main components in the development of rice cultivars with RYMV disease resistance. The paper provides a comprehensive review on the genetic variability of the RYMV, its epidemiology and control measures, and the gene action responsible for RYMV resistance.The review also summarises complementary genomic tools useful in RYMV disease resistance breeding.Successful breeding of rice for RYMV resistance depends on the availability of genes for stable resistance, knowledge of the genetics of the host and, the availability of efficient phenotyping and pathotyping methods, and understanding of the genes involved and their pattern of inheritance. Information presented in the review can serve as a reference guide for rice breeding emphasising RYMV resistance, high yields and farmers-preferred traits.