Grapevine downy mildew (DM), caused by the biotrophic oomycete Plasmopara viticola (Berk. & Curt.) Berl. & de Toni, is one of the most important plagues affecting viticulture, especially in humid climates. This pathogen is able to differentially attack leaves and grapes and is currently controlled with the massive use of fungicides, which has considerable economic costs as well as a relevant negative impact on environment, human health, and public perception. The use of resistant grapevine varieties, obtained through breeding programs, is a cost-efficient way to control grapevine DM. Most of the in vitro tests developed so far for DM resistance assessment are focused on leaf disk bioassays. This led us to consider that these tests might not always represent a proper evaluation and prediction of the disease impact on grape bunches and therefore on final production and quality. At first different experimental tests were carried out on the bunch to develop a new infection and phenotyping method. At a later stage this method was applied to screen several genotypes, in parallel with the traditional leaf disks phenotyping approach. In this way we were able to compare the different responses to the pathogen between leaf and bunch collected in untreated field. Here we present the detailed protocol that will be applied on some specific genotypes of interest to investigate whether the plant-pathogen interaction is organ-specific.