Development of a novel phenotyping method to assess downy mildew symptoms on grapevine inflorescences

Grapevine downy mildew (GDM), caused by the oomycete Plasmopara viticola (Berk. & Curt.) Berl. & de Toni, is one of the most important plagues affecting viticulture, especially in temperate rainy climates. P. viticola reduces fruit quality and yield, either by direct infection of berries or as a result of the reductions in photosynthesis and plant vigor caused by leaf infections. GDM control is based on the repeated and massive use of fungicides, leading to problems such as environmental pollution, development of resistance and residual toxicity. The use of varieties showing durable resistance to GDM is an alternative and promising strategy to control the disease. Nevertheless, most of the in vitro tests developed so far for GDM resistance are focused on leaf disk bioassays that not always allow for a proper evaluation and prediction of the disease impact on grapevine inflorescence/bunch and therefore on final yield and wine quality. In this work, we first improved the annotation procedure of foliar resistance/susceptibility under controlled conditions (optimized OIV descriptor 452-1). Second, we developed a new in vitro phenotyping method - from infection to symptom evaluation - for GDM resistance assessment on grapevine inflorescence, identifying the most responsive phenological stage (developed and proposed OIV descriptor 453-1). Thus, at this stage we screened several genotypes, in parallel with the optimized leaf disk bioassay, to compare the different pathogen responses between leaf and inflorescence collected from plants in untreated field. Finally, we validated our results performing the same GDM resistance assessment also on organs detached from fruiting cuttings grown in phytotrone.