Effect of seed circulation systems on the spread and diversity of Xanthomonas phaseoli pv. manihotis infecting cassava crops of the Colombian Caribbean

Cassava is a lifeline for millions of people across the tropics, but its productivity is at risk from a disease called Cassava Bacterial Blight (CBB). In this study, we explore how the everyday practices of farmers exchanging cassava cuttings—a long-standing tradition—can shape how the disease spreads and evolves in the Colombian Caribbean. By combining knowledge from socio-cultural studies, plant pathology, and genetics, we reveal an often-overlooked reality: plant diseases can spread through human activities. Our findings shed new light on CBB epidemiology and emphasize the importance of incorporating farmers' practices and local economies into the development of effective, sustainable disease control strategies. Although our knowledge about the prevalence of CBB and the diversity of Xpm populations has deepened in recent years, the role played by the seed circulation system on pathogen dispersal is still unknown. In this paper, our objective is to determine the influence of the social network shaping seed circulation on the dispersion and diversity of the pathogen at the local scale. Cassava is particularly interesting as planting material is often scarce at the time of the planting season, and diseases may not be fully visible, making it hard to control the phytosanitary condition of the propagative material. We used interdisciplinary methods based on ethnoecology, epidemiology, ecology, social network analysis, and genetics that included semi-structured interviews with farmers, coupled with molecular genotyping of cassava plants and Xpm. Our findings highlight two main effects of local seed circulation on Xpm spread and diversity: (1) a high diversity of the pathogen at the scale of the evaluated village, and (2) greater genetic similarities of the pathogen between farmers that exchanged cuttings, regardless of the geographic distance between the fields. This study reveals the multifactorial nature of Xpm diversity and spread within a single village. By combining genetic, agricultural, and social data, it shows the value of interdisciplinary approaches to understand plant pathogen dynamics. The findings support the design of locally adapted disease control strategies. Unlike studies focused solely on genetic data, this work emphasizes the importance of integrating sociocultural factors, particularly for seedborne pathogens and crops where seed circulation systems are a determining factor.