Blackleg of canola (oilseed rape, Brassica napus) is caused by two closely related fungal species, Leptosphaeria maculans and L. biglobosa. In Australia, with a few rare exceptions, blackleg is caused by L. maculans, whereas in Europe and north America, both species coexist. This paper reviews factors influencing the distribution of L. maculans and L. biglobosa with particular emphasis on the role of dispersal on a world-wide and local scale. The pathogens can be spread to new areas by seed movement or by wind-dispersed ascospores from fruiting bodies on crop debris. Although many ascospores travel less than 1 km from the source, their aerodynamic properties suggest that some may travel considerable distances. This may explain why the breakdown of host resistance based on major genes can be widespread. Slight differences occur between L. maculans and L. biglobosa in tissue colonisation, survival on crop debris and the timing of maturation and release of airborne ascospores. These subtle differences in biology explain how the pathogens occupy different niches on the same host and so can sometimes coexist, while large differences in their geographical distribution can also occur.