Knowledge of the extent and distribution of variability within M. fijiensis is necessary for efficient genetic improvement of banana for a durable resistance to black leaf steak disease. The population genetic structure of M. fijiensis has been studied at a global, country and field scale using molecular markers. These studies have shown that populations of M. fijiensis can maintain a high level of genetic diversity at a small scale, and that recombination plays an important role in this pathogen. The Australasia Southeast Asia region is the centre of M. fijiensis diversity. The founder effects that have accompanied the introductions of M. fijiensis to Latin America, the Pacific Islands, and Africa have led to a reduction of genetic diversity in these regions. The implications of these results for resistance breeding and disease management will be discussed. Population analysis of M. fijiensis should include pathogenic variability studies. The characterization of disease resistance in banana germplasm is also essential for banana breeding. To this order, we are developing an inoculation technique for M. fijiensis, using banana leaf pieces. Similar infection patterns and symptom evolution to that observed under field conditions have been obtained. All of the different stages of the M. fijiensis life cycle have been reproduced in the leaf piece assay, and resistant phenotypes of cultivars were expressed. The first results obtained of M. fijiensis pathogenic variability, and the characterization of resistance within banana, using this technique will be presented