1Rice yellow mottle virus is a scourge which – for once – spreads slowly. In order to be contaminated, a rice plant must come into contact with a plant affected by the virus or interact with beetles capable of transmitting the virus. Certain farming practices such as transplantation can also accelerate the spread, which still remains rather slow. Consequently, the virus has had plenty of time to evolve and differentiate into several strains of variable virulence. In the late 1990s, two teams of researchers initiated research into this disease. The first team attempted to select rice plants resistant to rice yellow mottle virus, while the other focused on the virus, more specifically its genetic diversity.

Rice variety sensitive to the rice yellow mottle virus.

2To this end, the second team analysed the RNA of viruses collected throughout Africa on different dates and showed that rice yellow mottle virus first appeared in East Africa, and evolved as it spread. As a result, every virus strain has its own distribution area. The characteristics of these strains were tested in laboratories. In 2018, this study led to the publication of a map outlining the distribution area and virulence of the various virus strains, as well as a guide for the appropriate deployment of the most resistant rice varieties, for example by avoiding regions with the highest risks of the virus breaking this resistance and attacking these varieties: in Burkina Faso, Togo, Benin, Niger and Nigeria, researchers discovered a particularly virulent virus strain, against which no known resistance gene is durably resistant. Moreover, this mapping work revealed anomalies in the circulation of certain strains, which seem to disperse more quickly than others and expand their distribution area.

Manual transplantation of young rice plants grown in a nursery, Tanzania.