Downy mildew is a fungal-like disease caused by Plasmopara viticola. The disease is most severe during seasons that are excessively wet and warm. All Vinifera vitis cultivars are susceptible to downy mildew. Susceptibility within North American species (Vitis labrusca) ranges from highly susceptible to resistant. Leaf symptoms vary depending on the age of the leaf and infections occur throughout the growing season. Most infection occurs during the period from early bloom through 3 to 4 weeks after bloom. Severe infections can lead to complete defoliation. The pathogen overwinters as dormant spores in infected leaves on the ground.
Signs and Symptoms
Young leaves are very susceptible to downy mildew. Initial infections appear as small, light yellow to green, oily spot (lesions) on the upper surface of the leaf. As the disease develops the spots enlarge and turn yellow (image on left; photo courtesy- apsnet.org) and have an irregular shape to them. Early downy mildew leaf symptoms are often confused with powdery mildew. As the leaf ages, the spots turn brown (necrotic) and become crusty (center image, photo courtesy- nicolewarduk.blogspot). Lesions are usually restricted in size by the leaf veins. During periods of wet weather and high humidity, masses of spores form on the underside of the leaf lesion. Lesions with spores appear as a dense white mass (image on right, photo courtesy- agric.wa.gov.au)
Initial infection spots
Necrotic spots and crusty edges
Masses of spores on underside of leaf
Infected shoots, stems (pedicels), tendrils and young flower clusters are curled and thicker than non-infected tissue (image on the left; photo courtesy- apsnet.org). Masses of white, down-like spores will form on infected shoots, stems (image on the right, photo courtesy- plantmanagementwork.org), and tendrils. When the disease is severe the entire cluster can be killed.
Curling due to an infected shoot
Masses of spores on infected shoots
Young infected berries turn brown, are soft and shatter easily (image on the left; photo courtesy- agric.wa.gov.au). Under humid conditions, the berries will be covered with downy white spores (image on the right; photo courtesy- fruit.cfans.umn.edu). Infected berries will never mature normally and will drop off of the cluster leaving a stem scar.
Shattered infected berries
Berries covered with downy white spores
Downy Mildew Disease Management
All cultivars of Vitis vinifera are considered susceptible to downy mildew, although cultivars such as Chardonnay, Pinot Noir, and Sultana are considered more susceptible than Cabernet Sauvignon and Semillon.
Several North American species show resistance to downy mildew (e.g. V. labrusca and V. rotundifolia). Interspecific hybrids of V. vinifera and the North American species have yielded cultivars with good wine-grape qualities and greater resistance to downy mildew. Wine consumers prefer the known varietal wines over the less-known and sometimes fruitier hybrids.
Practices that improve air circulation within the canopy such as shoot positioning and leaf removal and weed control are important to minimize warm and humid conditions that favor pathogen growth. Maintain a healthy plant by following good growing practices, more information can be found at Midwest Grape Production Guide.
Sanitation is a critical component of an integrated program to manage downy mildew, especially for organic and home garden growers. Strategies that prevent overwintering spores from reaching developing vines in the spring are very important. These include:
- Remove and destroy dead leaves and diseased fruit from the vines and the ground surrounding the vines after leaf drop.
- Cultivate and cover plant debris on the ground with soil after bud break to prevent spores from splashing onto the vines.
Downy mildew can be effectively managed by properly timed and effective fungicides. Fungicides should be applied as soon as possible if temperatures are between 50ºF and 85ºF and an accumulative rain event.
For commercial wine grape growers, there are many effective fungicides for controlling downy mildew. Commercial growers should consult the most current issue of the Midwest Fruit Pest Management Guide for up to date fungicide recommendations.
Homeowners can use Bonide Captan (50% captan) to help with downy mildew disease management. Captan is most effective when disease pressure is low and when used in combination with cultural and sanitation practices. Captan should NOT be mixed with oils or alkaline (high pH) materials such as hydrated lime.
There are currently no biocontrol products available that can effectively manage downy mildew.