The availability of fast, reliable and non-destructive methods for the analysis of pathogen development contributes to a better understanding of plant–pathogen interactions. This is particularly true for the genetic analysis of quantitative resistance to plant pathogens, where the availability of a method allowing a precise quantification of pathogen development allows the reliable detection of different genomic regions involved in the resistance. Grapevine downy mildew, caused by the biotrophic Oomycete Plasmopara viticola, is one of the most important diseases affecting viticulture. Here we report the development of a simple image analysisbased semi-automatic method for the quantification of grapevine downy mildew sporulation, requiring just a compact digital camera and the open source software ImageJ. We confirm the suitability of the method for the analysis of the interaction between grapevine and downy mildew by performing QTL analysis of resistance to downy mildew as well as analysis of the kinetics of downy mildew infection. The non-destructive nature of the method will enable comparison between the phenotypic and molecular data obtained from the very same sample, resulting in a more accurate description of the interaction, while its simplicity makes it easily adaptable to other plant–pathogen interactions, in particular those involving downy mildews.