Abstract: The ascomycete Guignardia bidwellii is the casual agent of the black rot of grape.Native to North America introduced to Europe in the early 19th century and today regularlyobserved in France, Italy, Germany and southern Switzerland. The black rot is not generallyconsidered as an important disease but it is able to induce heavy crop losses particularly inregions characterised by weather conditions favourable to Plasmopara viticola. Present studiesfocused on the genetic structure of the pathogen population, to get a better understanding of theepidemiology and life cycle of the disease. We successfully extracted and amplified DNA frommummified berries and developed six SSR markers for genotyping of G. bidwellii isolates. TheSSR markers were used to study the pathogen populations of two Merlot vineyards in France anda mixed variety vineyard (Merlot, Regent, Isabella, Solaris, Bianca and Chambourcin) insouthern Switzerland. All three populations were related and shared common SSR alleles. Genediversity was similar for the two French populations, and the Swiss population was nearly clonalin a genotype frequently observed in France. Only a few private genotypes were found, thereforethe populations differed mainly in genotype frequency. Based on our data we challenge theimportance of sexual reproduction for the propagation of the fungus. The low genotype and allelediversity observed could be explained either by a generally low genotype polymorphism inEurope or by a low SSR marker polymorphism. More research is therefore needed to assess SSRmarker polymorphism on an American population, to develop additional SSR markers for ahigher genotype resolution and to understand an eventually difference in the genotype frequencybetween leaf and berry infections.