BACKGROUND and OBJECTIVES Many crop diseases are propagated mechanically through infected stem cuttings that move along farmers' informal seed exchange networks. In Africa, the Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD) are major threats to regional food security and cause vast economic losses to staple crops. CMD and CBSD both originated from East Africa but have been steadily expanding westwards towards Central and West Africa (Legg et al., 2011). Successful control of crop pandemics requires a better understanding of the social factors that can enhance the spread of viruses through open and unrestricted seed exchanges or, on the contrary, slow down the adoption of pathogen-resistant cultivars through tighter social control of local seed systems. MATERIAL and METHODS Using a phylodynamic approach, we investigated how the spread of plant pathogens is influenced by local seed systems, by analysing spatial patterns of genetic variation in the African cassava mosaic virus (ACMV) in Gabon, central Africa. Viral diversity was characterized through DNA sequencing of the replication associated protein (Rep) open reading frame on the ACMV genome. Because genetic relationships between haplotypes not solely represent evolutionary ties but reflect also the movements of viral strains through regional networks of seed exchange, we analysed regional patterns of ACMV diversity not only from the point of view of population genetics but also from the point of view of social network analysis. RESULTS In small-scale farming communities, circulation of crop landraces is often channelled by social structures which determine the connectivity of farmer populations and, in extenso, favour or limit germplasm exchanges among communities (Delêtre et al., 2011). We show that social structures leave distinctive molecular signals on intraand inter-population levels of DNA polymorphism in virus metapopulations, which reflect in the topology and statistical properties of