Plasmopara viticola, commonly known as grapevine downy mildew, is a significant threat to grapevines worldwide. This organism can cause substantial reductions in grape yields and fruit quality, leading to financial losses for growers worldwide. First observed in the United States in 1834, it spread to Europe in 1878, causing widespread epidemics and yield losses, and is now found globally.
Understanding Plasmopara Viticola
Plasmopara viticola is classified as an oomycete, also known as a water mold, distinguishing it from true fungi. While oomycetes share some similarities with fungi, such as filamentous growth and spore production, they belong to a different phylogenetic group, the Stramenopiles, which also includes brown algae and diatoms. This distinction is important for control strategies, as fungicides targeting true fungi may not be effective against oomycetes due to differences in cell wall composition and metabolic pathways. Oomycetes, including P. viticola, possess cellulose in their cell walls, unlike true fungi which primarily have chitin, though P. viticola has also been found to regulate chitin synthesis and possess chitin in some structures.
Plasmopara viticola is an obligate biotroph, meaning it only grows and reproduces on living host tissue. Its host range is primarily limited to species within the Vitis genus, including cultivated grapevines (Vitis vinifera) and wild or ornamental grape species. This pathogen is considered one of the most destructive diseases in Europe and the eastern United States, causing substantial production losses that can reach up to 75% in humid grape-producing areas. The economic impact of downy mildew can range from approximately 2000 to 4250 euros per hectare in affected vineyards.
Identifying the Disease
The initial symptoms of Plasmopara viticola infection appear on the upper surface of young leaves as small, yellowish, translucent spots, known as “oil spots.” These spots are round or irregular and may eventually turn brown as the tissue dies. As the disease progresses, especially under humid conditions, a fuzzy, white, downy growth of sporangia becomes visible on the underside of these leaf spots. This white growth is a diagnostic feature of grapevine downy mildew.
On young shoots and tendrils, the disease can cause a water-soaked appearance, which then turns brown or black, distorting and eventually killing them. Infected young berries may also exhibit a grayish-white, downy growth, similar to that on leaves. As berries mature, they become resistant to direct infection, but if infected when young, they can shrivel, turn brown, and become susceptible to secondary infections. Early detection of these symptoms is important for timely intervention and management.
How the Disease Develops and Spreads
The life cycle of Plasmopara viticola begins with overwintering oospores, thick-walled, dormant spores found in fallen leaves and soil. In the spring, when temperatures are above 10-11°C and sufficient rainfall is present, these oospores germinate to produce macrosporangia. These macrosporangia then release motile zoospores that swim in water films.
Rain splashes can carry these zoospores from the soil onto the green tissues of grapevines, where they infect the plant through natural openings called stomata. After successful infection, within 7 to 10 days, the characteristic yellow “oil spots” begin to appear on the leaf surface. During periods of warm temperatures, around 25°C, and high humidity or dew, new sporangia are produced on the underside of infected leaves. These sporangia are then dispersed by wind or splashing water, leading to rapid spread of the disease to uninfected plant parts, including leaves, shoots, inflorescences, and berries. This initiates repeated cycles of infection as long as environmental conditions remain favorable.
Protecting Grapevines
Protecting grapevines from Plasmopara viticola involves a combination of strategies to manage the disease. Cultural practices play a significant role in reducing infection risk. Proper pruning and canopy management improve air circulation within the vine, helping to dry leaf surfaces and reduce humidity that favors pathogen growth. Selecting planting sites with good air drainage and sunlight exposure also helps create an environment less conducive to downy mildew.
Sanitation practices, such as removing and destroying infected plant debris (especially fallen leaves where oospores overwinter), help reduce primary inoculum for the following season. Planting grapevine varieties with some resistance or tolerance can decrease disease severity. Chemical control involves the timely application of fungicides, such as copper-based sprays, which have been used since the 19th century and remain a common treatment. An integrated pest management (IPM) approach, combining cultural practices, resistant varieties, and judicious use of fungicides based on disease forecasting models, offers the most sustainable way to manage grapevine downy mildew.