Grapevine downy mildew is one of the most dangerous diseases affecting vineyards. Caused by the fungus Plasmopara viticola, it is native to North America and was accidentally introduced to Europe in the 1870s through imported American vines, which are resistant to P. viticola; by contrast, European grapevines have proven to be very susceptible to the disease.

This grapevine disease causes significant damage, including reduced yields, lower grape quality and weakened plants, leading to knock-on effects in subsequent years. It is still a significant problem, and 2023 was a critical year for Italian vineyards due to its impact. Today’s blog article is all about grapevine downy mildew: what it is, how it develops and how to prevent it spreading to your vineyard.

Symptoms of grapevine downy mildew

What are the symptoms of grapevine downy mildew? The disease attacks all green parts of the plant. Here are the tell-tale signs by which you can recognise it:

• On young leaves, translucent yellow spots called oil spots appear, followed by a characteristic whitish mould that forms on the underside of leaves.
• The grape clusters darken, become covered in mould and then shrivel.
• The most developed bunches turn brown and then dry out, without visible mould growth: in this case we are talking about larval downy mildew, which is typical of late attacks, i.e. in summer.
• On older leaves, usually at the end of the season, small polygonal spots form, confined by the leaf veins (mosaic downy mildew).
• The vine shoots turn brown and dry out, and can also become distorted or develop cracks, from which mould grows.