Prevention of Mildew Disease in Grapevines is Still the Best Defense

Learn about the causes of downy mildew and powdery mildew in grapevine, and how to manage mildew with TerraviewOS.



Ohio wine country is blessed with rich and fertile soil thanks to a beautiful climate from glaciers that formed the Great Lakes. So when an Ohio winegrower lost nearly 80% of the estate’s 50-acre crop four years ago, the news sent shock waves to over 280 commercial vineyards operating in the Midwestern state. The reason? Unchecked downy mildew. It took another two years before the Ohio vineyard could recover from the damage.

Mildew disease is commonplace, but late detection can be severely harmful, not just in the current season, but the next season as well. All Vitis vinifera varieties in the world have become susceptible to powdery mildew and downy mildew diseases. The good news is the methods for treating them have evolved and strengthened as well.

Powdery mildew, or Erysiphe necator, was first discovered in 1845 in England by botanist Edward Tucker who observed that a plant disease in the United States had made its way to the U.K. Tucker found the plant disease had a molecular structure similar to the peach mildew he had treated in earlier years and successfully eliminated with sulfur.

The susceptibility of the various plant parts to powdery mildew infection became obvious to the casual farmer, as the vines changed through each season of the phenological cycle. The ascospores of cleistothecia emerged in the spring with rainfall, sprinkler irrigation, or fog, when shoots grew at temperatures of 18-30°C (64-86°F). Conidial production—asexual propagules that appear as a white powder dispersed by the wind—occurred in 7-10 days after primary infection and continued throughout the season as long as moderate temperatures persisted. In other words, the cleistothecia infected buds midsummer the previous growing season and quickly established hot spots of disease in the following season. This phenomenon was more common in regions that experienced milder winters, whereas regions with harder freezes killed the fungus in the buds.

Today, farmers understand the science of dormant mycelium spores and how these spores have everything they need to infect the plant. However, if they cannot infect a plant in approximately 24 hours, the spores die and the disease will not progress. The environment plays a huge role in how long it takes for a spore to infect the plant and can drastically affect how quickly or successfully powdery mildew becomes established each season. However, while most people associate dampness with fungal diseases, humidity isn’t a vital factor in the spread of powdery mildew. The conidia can germinate under dry conditions, and rain can hinder germination.

Powdery mildew has an equally destructive cousin called downy mildew, or Plasmopara viticola. Like powdery mildew, downy mildew also made its way to Europe from the United States after its discovery in 1834 by American botanist Lewis David de Schweinitz. Its impact on yield and production of grapes that year, especially in France, was swift and destructive, with some farmers losing up to 50% of their crops.

Within just a few years of the pathogen’s introduction, the French experimented with grafting, attempting to transplant root stock as an offshoot from vines that had coexisted with the disease in North America to their own vines in order to produce a more resistant strain of grape. Symptoms of downy mildew have since included necrosis of the stem or shoot, brown spotting (lesions), and yellowish-green tips of the leaves. Grapes may exhibit sporangia, the cells from which spores are produced, appearing as white-to-gray coating on the outer surface. After suitably warm, humid nights, the grayish-white sporangia will appear on the underside of the leaves and other infected plant parts, turning the edge of the leaves downward, hence its name.

Downy mildew has a specific set of environmental conditions to reproduce and infect its host that comprises a warm, moist, and humid environment. The earliest symptom of downy mildew is a cluster of yellow, oily round spots surrounded by brownish-yellow halos, which smothers the leaves 5-7 days after infection. The brownish halo fades as the oil spot matures. Interestingly, the oil spots are yellow in white grape varieties and red in some red grape varieties. Downy mildew is highly destructive in all grape-growing areas of the world where there is spring and summer rainfall at temperatures above 10ºC (50ºF). If the conditions are right, large numbers of oil spots may develop and coalesce to cover most of the leaf surface.

As July weather forecasts roll in, canopy management, chemical usage, and vineyard monitoring are paramount in stemming the tide of fungal infection. Let’s take a look.

Canopy Management:

Canopy density is a critical component of powdery mildew management. When excess shoots grow because of cold damage to primary buds, or when primary and lateral shoots explode due to wet springs, disease pressure increases. Opening up the canopy (particularly the fruit zone) is protective because sunlight reduces disease pressure. Climate change increases disease pressure by causing the period that is best for fungal growth to start sooner and last longer. Canopy opening also enables penetration of the leaf and fruit surfaces by fungicides.

Another factor crucial to canopy management is canopy architecture/vine spacing and how the vineyard’s layout can affect the flow of air. Canopy architecture can influence day and night vine temperatures because of relative humidity and sunlight exposure. The tighter the vine spacing is, the harder it is for the sun to penetrate with its radiation. Vines that are too close together also require multiple hedging passes, which stimulates the growth of lateral shoots and susceptible leaf tissue that shade fruit zones and compromise the integrity of the grape quality. While shoot thinning, leaf removal, and irrigation strategies can influence the canopy architecture, other factors, like cold and precipitation during the dormant season, can influence canopy development in the following season.

Chemical Usage

The main chemical treatment for powdery and downy mildew diseases is still sulfur, a natural element that is toxic to fungus, as a wettable powder formulation. For organic gardeners, sulfur is an important fungal disease-control product because of its low toxicity to humans. Since it works as a contact fungicide, it needs to cover all the green areas of the vine and must be re-applied at regular intervals. Sulfur has several mechanisms for dealing with the fungus so that fungal resistance to treatment doesn’t develop. In some areas, it needs to be applied continuously during times of peak disease pressure to stem inoculum buildup, especially when the environment is highly conducive for fungal growth.

Perhaps the greatest difference between organic and conventional production systems is that organic growers may not use synthetic conventional fungicides. Sometimes, organic growers limit themselves to the use of inorganic fungicides such as sulfur (elemental sulfur and lime-sulfur) or copper fungicides (Bordeaux mixture and fixed copper products). In addition, they benefit from several new alternatives such as HML32, a new fungicide approved for organic farming. Most of these alternatives are preventive, but some also have eradicant action. HML32 is a potassium-based liquid soap premixed with potassium bicarbonate. When it is mixed with a copper fungicide (such as Nordox) it is quite effective at knocking back an existing powdery infection.

Vineyard Monitoring

TerraviewOS can help predict mildew using the Vine Click app, which identifies diseases that are present in a vineyard, as well as their location. The app uses advanced machine learning algorithms based on thousands of photos to predict diseases simply by capturing images.

Precision viticulture, like intelligence gathered by using MAXAR’s True30 satellite imagery, can provide viticulturists and winegrowers an idea of the canopy cover of their vineyards to plan corrective action in advance. Additionally, multiple drone flights for clients such as Bodegas Ayuso, La Rioja, and Marta Mate vineyards in Spain provided detailed imagery of their canopy architecture for actionable and corrective insights. TerraviewOS dashboard displays the aerial images processed to show Normalized Differentiated Vegetation Index (NDVI), which helps with canopy cover management.

Alerts & Reccomendations

TerraviewOS also provides Alerts and Recommendations when environmental circumstances are conducive to mildew growth. With the Work Order module, treatments can be prescribed and progress can be monitored to ensure appropriate actions are taken. This assists the viticulturists to keep track by recording when, where, and to whom tasks are assigned or reporting and vineyard management.

Got questions about how TerraviewOS can help you combat mildew? Click here to sign up and learn more.