Abstract
Cassava (Manihot esculenta Crantz) is extensively cultivated all over the tropics, as its tuberous root is a major source of carbohydrates for hundreds of millions of people worldwide. Because of its tolerance to heat and water stress, cassava is thought to be resilient to climate change. In Colombia, cassava is one of the most cultivated food crops, ensuring the subsistence of many smallholder family farmers. The Caribbean region contributes to about 50% of the national production, mainly through small-scale cultivation by local communities that not only use it as part of their traditional diets, but also as a source of income.Cassava is threatened by several pests and diseases that can significantly reduce the crop yield, thus creating socioeconomic issues to small-scale farmers that depend on their production for survival. Among the main diseases affecting cassava production is Cassava Bacterial Blight (CBB), which is caused by Xanthomonas phaseoli pv. manihotis (Xpm). Xpm is a systemic Gram-negative bacterium that penetrates host leaves through natural openings or wounds, and can move into distal leaves and stems upon colonization of the plant vascular system. The pathogen can spread via contaminated tools used to prepare stem cuttings at planting time, and to cut stems upon harvest. Exchanges of infected stem cuttings can disperse the pathogen among fields. Moreover, farmers recognize CBB as a concern because of the lack of knowledge to control it, but it is still not given the necessary attention since farmers lack training for CBB identification or have no interest in stopping it because they underestimate its impact on yield.There is quite detailed knowledge about the genetic diversity of the Xpm population, its evolutionary dynamics and the genes involved in the virulence of the pathogen. However, aspects such as the impact of farming practices and the sociopolitical context on Xpm epidemiology have yet to be determined. In this sense, the main objective of this PhD thesis was to investigate, through multidisciplinary approaches from the social and natural sciences, the role played by local knowledge, agronomic practices, and socioeconomic factors on the occurrence of CBB, at the scale of the village Villa López located in the Caribbean region of Colombia.Our findings show that (i) CBB occurrence strongly depends on land property issues, likely because of an inadequate phytosanitary control during acquisition of cuttings when farmers are forced to rent the land. (ii) The cassava seed circulation system is strongly impacting the spread and diversity of Xpm at the scale of Villa López. (iii) There is a strong positive correlation between the changes of agricultural practices and the occurrence of CBB in the village of Villa López. Additionally, (iv) the agrarian history of the cassava cultivation in the region and the effects of land tenure conflicts on the spread of CBB over time were registred, through a participatory method made together with farmers. This information can be a key element to improve our understanding of the pathogen population genetic structure and its prevalence worldwide, and, at the same time, it provides necessary information for early detection and sustainable control of CBB in cassava crops at the local scale.