Distinctive Symptoms and Signs of Downy Mildew on Cold-Climate Wine Grape Cultivars

Downy mildew, caused Plasmopara viticola, is a major disease affecting grape production worldwide. While the symptoms and signs of downy mildew on European wine grapes (Vitis vinifera) are well documented, minimal information is available regarding the symptoms and signs on cold-hardy cultivars that are crosses of V. vinifera and Vitis species native to North America. Referred to as “cold-climate cultivars,” these hybrids have been released over the past two decades and form the basis of a burgeoning wine industry in the northern United States. Most research on downy mildew of grape has involved V. vinifera, and consequently, many images used for educational purposes illustrate “classic” symptoms and signs on highly susceptible cultivars of V. vinifera. However, we have observed that such images can be inconsistent with symptomology in the cold-climate grape hybrids, and there are differences in the symptoms and signs of downy mildew among cold-climate cultivars. These inconsistencies can result in incorrect diagnosis, which can lead to poor management decisions, including application of products that are not appropriate for the pathogen(s) present. Thus, the objective of the current guide is to describe downy mildew symptoms and signs on cold-climate cultivars, with special attention to features that are not documented in resources widely used for grape disease diagnosis.