1In 1985, a bacterium devastated all the cassava fields in Congo-Brazzaville, causing large-scale famine. Observed a few years earlier in Côte d’Ivoire, it was believed to have fallen down from the sky. No, Xanthomonas phaseoli pv. manihotis, or Xpm as it is commonly known, is from this earth, as demonstrated by researchers when they detected its existence in Latin America, the home region of cassava and probably that of the bacterium, where it also decimates plantations and spreads with cuttings. To grow cassava, farmers transplant stem cuttings. If the initial plant is infected, the risk of an epidemic situation is significant.

Bags of cassava semolina, Brazil.

2To try and reverse this trend, researchers created cassava nurseries in forest areas fairly isolated from one another. However, they also focused on the molecular mechanisms which determine the virulence of the microorganism. In the early 2000s, they sequenced the bacterial genome. One of the 5,000 genes discovered caught their attention as its inactivation completely prevents the bacterium from multiplying. This opened up new disease control options. Meanwhile, in 2018 they developed a high-performance molecular diagnosis tool to detect infected plants and prevent the spread of contaminated cuttings.

3Researchers also explored the genetic diversity of the microorganism to shed light on its ability to break the resistance mechanisms of cassava. To do so, they collected diseased seedlings in various African and Latin American countries, but also in herbaria, finding a cassava leaf which had been infected in 1821 in Argentina. This long-term research will improve knowledge of the pathogen’s historical and contemporary distribution area, with a view to ultimately increasing the understanding of its evolutionary history and predicting future epidemics.