Pseudoperonospora humuli might be an introduced species in Central Europe with low genetic diversity but high distribution potential.
The downy mildew pathogen Pseudoperonospora humuli is a major disease of cultivated and wild hop (Humulus lupulus) in many hop-growing areas of the world. Despite its frequent occurence and economic impact, so far very little is known with respect to its population structure, which would be a prerequisite for understanding population dynamics and for successful resistance breeding efforts. We collected a set of 70 P. humuli samples in five European countries (Austria, the Czech Republic, France, Germany, Italy) around the Alps, a high mountain range and a known major hurdle for dispersal, and examined them with microsatellite markers. For this purpose we developed a set of new markers and used them along with markers published for P. humuli by other authors. While the 11 microsatellites that were reported as polymorphic for North America did not show variation for the European samples, the five markers newly established in this study were variable and could be used to investigate the population structure of hop downy mildew in Europe. The overall genetic variation found in European populations was low, with only few heterozygous loci detected. Overall, 20 multilocus genotypes were observed, which clustered into 2 or 4 groups, depending on clustering criteria. We observed a random distribution of genotypes on the surveyed area in line with predominant selfing and long range dispersal. Thus, our investigations suggest that Pseudoperonospora humuli forms a large, potentially panmictic population with little local adaptation and frequent long-distance exchanges. This suggests that a wide array of Pseudoperonospora humuli strains should be used in any resistance breeding programs. In addition, the low degree of variation in Europe in contrast to North America suggests that, in line with its report in Europe as late as the 1930's, hop downy mildew might be an introduced species in Europe. Whether the introduction occurred from North America or from East Asia needs to be clarified in future studies.