Genetic diversity of isolates of Leptosphaeria maculans from a canola (Brassica napus) paddock in Australia

Amplified fragment length polymorphism analysis was used to differentiate isolates of Leptosphaeria maculans (Desm.) Ces. et de Not. which were collected, using a hierarchical sampling method, from a commercial canola (Brassica napus L.) paddock at Lake Bolac, Victoria, Australia. Many polymorphisms were found between individual isolates, but these did not allow differentiation into groups corresponding to the sampling hierarchy. Six isolates from a historical blackleg collection were included for comparison with the hierarchical sample. Although these isolates were similar, findings based on non-metric multi-dimensional scaling (NMDS) and unweighted pair group with arithmetic means (UPGMA) show that they do not group exactly within the hierarchically sampled isolates. Diversity indices and AMOVA give a conflicting representation of the diversity within the paddock. AMOVA suggests that there is only very weak hierarchical structure of diversity and there is as much diversity within any of the one metre sampling sites as there is between the six historical isolates which came from hundreds of kilometres apart. However, comparison of diversity estimates suggests that there are significant differences in between-isolate diversity between the sites, although these differences are small. It would appear that in order to gain an effective representation of the amount of diversity in any one location as part of a large-scale study of diversity across the country, a complex hierarchical sampling strategy such as the one employed for this study is not required.