IRD researchers and their partners in Benin, Burkina Faso and Côte d’Ivoire have, for the first time, described new diseases affecting rice and cassava, two basic food crops in the many countries in the Global South. These diseases are caused by Xanthomonas genus bacteria that attack the leaves and stalks and they are spreading in West Africa as rice growing expands and uses more intensive methods.
In rice, this bacterial disease is also known as ‘bacterial leaf spot’ and is one of the most serious disorders affecting the plant. It is found in most rice-growing regions across the world, except for Mediterranean areas, and causes considerable damage with up to 30% yield loss. It was first reported in Africa in the 1980s, with cases in Madagascar, Senegal and Nigeria. More recently, with the expansion of rice-growing and the introduction of high-yield varieties over the last few decades, scientists have observed its emergence in West Africa, where rice-growing has increased massively since 2000. More intensive farming increases the risk of pathogen emergence.
The bacteria is now found in most regions and especially Mali and Burkina Faso where the researchers characterised it. The disease has also recently been reported in the north of Benin. Further investigations need to be carried out to measure how far it has spread in these countries.
The genetic analyses of the Xanthomonas strains collected in Mali and Burkina Faso show that these African strains are very diverse, and appear to be related to either the imported Asian strain or an older African strain. Their genetic evolution is also very rapid.
The researchers have also shown that the severity of symptoms varies greatly from one strain to another, regardless of place, period or type of plant on which they were collected. This suggests that less virulent strains live alongside highly virulent strains in the same environments.
Finally, scientists have observed the bacteria on different host plants, as was previously described in Asia. In addition to rice crops, it was found in weeds and perennial grasses, as well as in wild rice varieties. These plants are a natural reservoir for the disease, outside crop growing seasons.
In cassava, the disease caused by the Xanthomonas bacteria genus, known as ‘bacterial blight’, appeared in the middle of the last century in Brazil and has followed the expansion of cassava growing across the world. In fact, cassava is now the world’s sixth biggest food crop with more than 700 million people dependent on it for food safety. The disease affects the propagation material, i.e. the stem, and is the plant disorder that causes the highest losses, with up to 30% of harvests lost. Its presence has recently been confirmed by IRD researchers and their partners in Burkina Faso and Ivory Coast, where it was reported in 1979. However, the responsible pathogen had not been characterised until now. The researchers have used new genetic methods to confirm that it is in fact the Xanthomona genus bacteria. This result is worrying in that cassava growing is continuing to expand in different parts of the country and elsewhere in the sub-region.
Given the extent and severity of bacterial epidemics over the past few years in Africa, it is now crucial to develop disease-resistant rice and cassava varieties. Researchers have identified certain resistant rice varieties grown in Burkina Faso, together with the gene that gives the plant this characteristic. These results will help guide selection procedures towards relevant varieties to strengthen food safety in affected countries.