Mycosphaerella musicola is a fungal pathogen of banana which causes yellow Sigatoka, also known as leaf spot disease. Heavy infection by M. musicola may result in reduced yields and poor quality fruit. M. musicola is widespread throughout Australian banana plantations. The major banana production regions in Australia are located in north Queensland, southeast Queensland and northern New South Wales. A population genetics approach was taken to examine if different populations of M. musicola existed in different production regions; if the climatic conditions of each region affected the mode of reproduction in M. musicola; and the occurrence of gene flow among populations of M. musicola. Single copy RFLP probes were used as genetic markers to examine 363 isolates of M. musicola. Isolates were sampled from 16 plantations and 12 sites. Hierarchical sampling revealed that isolates from adjacent or close plantations belonged to the same genetic population, while more geographically distant plantations each formed separate populations. High levels of genotypic diversity and very low levels of gametic disequilibrium were observed for each population, indicating that sexual reproduction has a major role in the genetic structure of M. musicola. Genetic differentiation studies revealed a high degree of genetic similarity between populations from the northern and southern production regions, indicating that gene flow has occurred. Ascospores carried on wind currents may account for some of the gene flow. A more probable explanation is the movement of infected plant material over time. In contrast, high levels of genetic differentiation were observed for the populations sampled outside of the major production regions. These populations were sampled from small plantations in isolated regions; thus reduced gene flow and genetic drift may account for this differentiation. (Author's abstract)