Dispersal processes of fungal plant pathogens can be inferred from analysis of spatial genetic structures resulting from recent range expansion. The relative importance of long-distance dispersal (LDD) events vs. gradual dispersal in shaping population structures depends on the geographical scale considered. The fungus Mycosphaerella fijiensis, pathogenic on banana, is an example of a recent worldwide epidemic. Founder effects in this species were detected at both global and continental scale, suggesting stochastic spread of the disease through LDD events. In this study, we analysed the structure of M. fijiensis populations in two recently (∼1979-1980) colonized areas in Costa Rica and Cameroon. Isolates collected in 10-15 sites distributed along a ∼250- to 300- km-long transect in each country were analysed using 19 microsatellite markers. We detected low-to-moderate genetic differentiation among populations in both countries and isolation by distance in Cameroon. Combined with historical data, these observations suggest continuous range expansion at the scale of banana-production area through gradual dispersal of spores. However, both countries displayed specific additional signatures of colonization: a sharp discontinuity in gene frequencies was observed along the Cameroon transect, while the Costa Rican populations seemed not yet to have reached genetic equilibrium. These differences in the genetic characteristics of M. fijiensis populations in two recently colonized areas are discussed in the light of historical data on disease spread and ecological data on landscape features.

We developed a total of 50 microsatellite markers for the three fungal pathogens causing the most important leaf spot diseases of banana: 32 loci for Mycosphaerella fijiensis are presented, and nine loci each for Mycosphaerella musicola and Mycosphaerella eumusae. All these loci were polymorphic within each species on a sample of isolates collected from various locations around the world. Within M. fijiensis and M. musicola, most of the loci tested (> 80%) in a sample of isolates from a single location in Cameroon were also polymorphic. Multiplex polymerase chain reaction systems were developed with 15 loci for M. fijiensis.