LAUSR

Downy mildew of grape, caused by Plasmopara viticola, is a global pathogen of economic importance to commercial viticulture. In contrast to populations in the northern hemisphere, few studies have investigated the population biology, genetic diversity and origin of the pathogen in Australian production systems. DNA was extracted from 381 P. viticola samples from Vitis vinifera and alternate hosts, collected via fresh and herbarium leaves from populations within Australia and Whatman FTA® cards from North America, Brazil and Uruguay. A total of 32 DNA samples were provided from a French population. The populations were genotyped using 16 polymorphic microsatellite markers. Representative samples from within Australia, Brazil and Uruguay were also genotyped to determine which of the cryptic species (clades) within the P. viticola species complex if any cryptic P. viticola populations were present. Our findings suggest the Australian and South American populations of P. viticola are more closely related to the European population than the North American, the reported source of origin of the pathogen. The Western Australian population had similarities to the South Australian population and the tight clustering of samples suggests a single introduction into Western Australia. P. viticola clade aestivalis was the only cryptic populations clade detected in Australian and South American populations. Analysis of the Western Australian population suggests it is reproducing clonally, but further research is required to determine the mechanism as to how this is occurring.