Epidemiology and spread of Leptosphaeria species (phoma stem canker) on brassicas

Leptosphaeria maculans and L. biglobosa, cause of phoma stem canker disease of brassicas, are related pathogens of brassicas that were originally considered as one species but occupy slightly different ecological niches and are now reproductively isolated. Both pathogens are naturally dispersed by air-borne ascospores, produced on infected crop debris, which may be dispersed over longer distances as a contaminant in internationally traded seed cargoes. Globally, the invasion by the more damaging L. maculans into areas where only L. biglobosa was present occurred in North America in the 1980s and Eastern Europe in the 1990s, whereas there are still areas of the world, such as China, where only L. biglobosa is present. In addition, the worldwide population of L. maculans is much less variable than that of L. biglobosa, which has populations related to particular geographic areas (Australia, Canada and the Indo-European continent) that should be considered as different species that diverged > 4MYA. Further evidence is provided by the massive invasion of the genome of L. maculans by numerous repeated elements; recent evidence suggests that divergence between L. biglobosa and L. maculans occurred 72MYA. As the environment and brassica cropping patterns alter in response to climate change, further evolution of these two Leptosphaeria species will occur, with possible increases in the range and severity of phoma stem canker epidemics.