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

Grapevine downy mildew (DM), 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 reduction in photosynthesis and plant vigor caused by leaf infections. DM 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 DM is an alternative and promising strategy to control the disease. Nevertheless, most of the lab tests developed so far for DM resistance assessment are focused on leaf disc bioassays. This led us to consider that these tests might not always represent a proper evaluation and prediction of the disease symptom extent on inflorescences/bunches and therefore on final yield and grape/wine quality. Therefore, based on the screening of nine Vitis hybrids, we developed a new lab phenotyping method to assess the disease extent on inflorescences at flower button phenological stage, along with a novel annotation descriptor. Secondly, we combined this approach with the optimized leaf disc bioassay and found a general positive correlation between organ DM resistance phenotypes. Finally, we found that Cabernet Cortis could be a model to study DM divergent dual (on leaf and inflorescence) epidemics.