The worldwide destructive epidemic of the fungus Mycosphaerella fijiensis on bananas started recently, spreading from Southeast Asia. M. fijiensis is an haploid and heterothallic ascomycete fungus and it spreads through three modes1. The wind-borne spread of ascospores produced during the sexual reproduction is believed to be limited to few hundred kilometers2. Conidia produced during asexual reproduction might be more involved in short-distance dispersal on the plant and to nearby plants. Populations and epidemiological studies of M. fijiensis have been undertaken (i) to provide information on the level and distribution of variability, (ii) to infer on dispersal process of M. fijiensis and (iii) to evaluate the relative importance of evolutionary factors on the pathogen. Population structure of M. fjiensis was analysed from global to plant scales using molecular markers3,4 The results indicate that a high level of genetic diversity is maintained at the plantation and plant levels. The loci were at gametic equilibrium in most of the samples analysed, supporting the hypothesis of the existence of random-mating populations of M. fijiensis, even at the plant level. Southeast Asia has the highest level of genetic diversity, supporting the idea that the pathogen originated in this region. Founder effects were detected at the global and continental scales. Genetic differentiation values between populations decreased with the geographical scale considered, from a high level at global scale (Fst = 0.52 between continents) to a non-significant level at the local scale (Fst = 0 between nearby plantations) (Figure 1). An isolation by distance analysis was conducted in Costa Rica and Cameroon within a production area (around 300 km long) to estimate gene flow and study the dispersal process of the pathogen (D. Coste and colleagues, unpublished results). A strong isolation by distance was detected in both countries, suggesting important dispersal of ascospores on short distances