Downy mildew on grapes

Grape downy mildew is one of the most significant diseases of wine and table grapes in temperate regions globally. Introduced to Europe from North America in the late 1800s, where it nearly destroyed the French wine industry, the disease remains a primary target of fungicide programs in commercial.

Grape downy mildew is one of the most significant diseases of wine and table grapes in temperate regions globally. Introduced to Europe from North America in the late 1800s, where it nearly destroyed the French wine industry, the disease remains a primary target of fungicide programs in commercial vineyards. For home gardeners, a single wet spring can defoliate grapevines and render the fruit crop unmarketable from a single season's epidemic.

I don't grow grapes at my Long Island property, so this guide draws on Cornell Cooperative Extension viticulture research and UC IPM publications.

The pathogen

Grape downy mildew is caused by Plasmopara viticola, an oomycete (not a true fungus) closely related to the pathogen causing potato late blight. Per Cornell Cooperative Extension, P. viticola overwinters as thick-walled oospores in infected fallen leaves on the vineyard floor. In spring, oospores germinate when temperatures exceed 50°F (10°C) and there is sufficient rainfall; they produce primary sporangia that infect grape shoots through stomates.