Grape Downy Mildew (Plasmopara viticola) – Biological Control, Symptoms and Development

Grape downy mildew or grapevine downy mildew, caused by Plasmopara viticola, is one of the most important diseases in wine and table grape production worldwide. The disease spreads rapidly under warm and wet conditions, infecting leaves, shoots, and young berries, often resulting in severe yield and quality losses. Infections during flowering and early berry development pose the greatest risk to crop performance. Early symptom detection, a clear understanding of the disease cycle, and vigilant monitoring of high-risk weather conditions are key to maintaining a healthy vineyard.

Underside of a grape leaf infected with downy mildew, with clearly visible sporulating lesions.

Easy to use
Minimal resistance
No chemical residues
No pre-harvest intervals

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Signs and Damage Symptoms
Management
Life cycle

Signs and damage symptoms of Grape downy mildew

Symptoms of grape downy mildew (Plasmopara viticola) vary according to the crop’s growth stage, environmental conditions, and infection pressure. While all green tissues of the grapevine are susceptible, characteristic ‘oil spots’ on the upper surface of young, actively growing leaves, followed by downy white growth on the underside, are typical early symptoms. Because visible symptoms usually appear 7–10 days post-infection (depending on temperature and humidity), the disease is often well-established before it can be visually detected in the field.

Leaf symptoms

Yellowish to light-green oil spots on the upper leaf surface, often angular and limited by leaf veins
White to greyish downy growth on the underside of leaves under humid conditions
Spots turning brown and necrotic as the disease progresses
Premature leaf drop in severe infections, reducing photosynthesis and vine vigor

Shoot and tendril symptoms

Brown to dark lesions on young shoots and tendrils
Distorted or stunted shoot growth
Increased susceptibility of infected tissue to secondary infections

Flower and cluster symptoms

Infection of inflorescences and young clusters, particularly around flowering
Browning, drying, and collapse of flower clusters
Poor fruit set and uneven cluster development

Berry symptoms

Young berries turning greyish-brown and shriveling
Infected berries remain small and fail to ripen
Clusters appear sparse due to early berry loss

First visible leaf symptom of grape downy mildew, showing the characteristic “oil spot”.

Early and severe downy mildew infection on a grape inflorescence during the pre-flowering stage.

Close-up through a hand lens showing active downy mildew sporulation on a grape inflorescence.

Grape bunch infection showing shriveled and dried flowers and young berries on the rachis.

Management of Grape downy mildew

Effective management of grape downy mildew (Plasmopara viticola) relies on a preventive and integrated approach. Since the pathogen remains latent before symptoms emerge, control strategies should focus on reducing infection pressure, limiting favorable conditions for disease development, and maintaining protective coverage on susceptible tissues during high-risk periods.

Cultural and preventive measures

Cultural practices play an important role in reducing disease pressure:

Improving air circulation through canopy management and pruning
Reducing leaf wetness by optimizing irrigation timing and method
Removing infected plant material and fallen leaves to reduce overwintering inoculum
Avoiding excessive nitrogen that promotes dense canopy growth

These measures help create less favorable conditions for disease development but are usually not sufficient on their own under high pressure.

Biological control of Grape downy mildew

Biological control plays an increasingly important role in the management of grape downy mildew (Plasmopara viticola), particularly within Integrated Pest Management (IPM) programs. The free-living amoeba Willaertia magna C2c Maky is known for its ability to produce a wide range of bio-active compounds involved in microbial interactions. When used as a lysate, these compounds support plant protection by acting both on the plant surface and by activating the plant’s natural defense mechanisms. This dual mode of action makes Willaertia magna–based solutions well suited for the preventive management of grape downy mildew.

Life cycle of Grape downy mildew

Plasmopara viticola is an obligate biotrophic pathogen of grapevines, characterized by sexual overwintering and rapid asexual cycles during the growing season. The pathogen overwinters primarily as oospores in fallen leaf litter, though in milder climates it can also persist as mycelium within infected buds and canes. In spring, oospore germination is triggered when temperatures reach 10°C in the presence of soil moisture, leading to the formation of macrosporangia that release motile zoospores within 24 hours.

These zoospores are dispersed via rain splash to the canopy, where they encyst and penetrate the host through stomata. Once inside the host tissue, hyphae extend intercellularly, forming haustoria to parasitize mesophyll cells. The latent period (the time from infection to visible symptoms) ranges from 4 to 21 days, depending on temperature; the most rapid development occurs on young foliage at optimal temperatures of 19-24°C.