Mildew and downy mildew on grapes in the vineyard

Powdery mildew (also called powdery mildew) and downy mildew on grapes are the bane of many winegrowers. In this article, I'll explain, recognize, prevent, and combat these fungal diseases. I hope that after reading this article, you'll have the strategies and tools you need to keep your vineyard free from a high level of (downy) mildew.

Harvesting healthy, ripe, and intact grapes in the fall is crucial for making a beautiful, flavorful wine. Unhealthy, damaged, and unripe grapes are a nightmare to vinify. With the information, tips, and my prevention plan in this article, you should be able to control powdery mildew. I wish you the best of luck.

The main difference between powdery mildew and downy mildew is that powdery mildew looks like spilled powder on the leaves, while downy mildew causes leaves to droop and develop yellow and brown oily spots. Furthermore, powdery mildew spores appear on both sides of the leaves. But downy mildew spores are only on the underside of the leaf. Furthermore, powdery mildew produces a thick web of white spores that cover the leaves, stems, and shoots, while downy mildew spores appear as a gray or white, fluffy layer on the underside of the leaves.

In short, powdery mildew ( Uncinula necator ) and downy mildew ( Peronospora viticola ) are two types of fungal infections in plants. Generally, powdery mildew is an ascomycete fungus (family of sac fungi), while downy mildew is an oomycete fungus (a water mold). Therefore, they are two different types of fungi that require a plan of action in the vineyard. In my prevention plan, I combat both fungal diseases year-round.

Every year, powdery mildew strikes our vineyards, but also cucumbers, lettuce, zucchini, nut trees, and dozens of other crops. The fungus overwinters in the vineyard in the old wood. You can tell by the yellow or black spots on the wood from the previous year. Therefore, in the autumn, after the harvest, a fungicide treatment is recommended in the vineyard (my recommendations can be found later in this article). You gain one point in your battle against the fungus before the next season begins, but the battle is far from over.

Powdery mildew thrives in warm, dry conditions. Despite the word "dew" in the name, the fungus thrives in dry conditions. This allows it to spread throughout your vineyard. Once it gets humid, the white, powdery coating appears on your leaves and berries. Between 15 and 27 degrees Celsius, the pest thrives. The white cast on your leaves prevents photosynthesis, and the white cast on your berries damages their cell structure, with all the consequences this entails when ripening. Berries can burst open, attract wasps, and harbor unwanted bacteria.

Downy mildew is caused by the fungus Plasmopara viticola. It's an unfortunately common and destructive disease in vineyards. Downy mildew appears in your vineyard under certain environmental conditions. Heat, high humidity, heavy rain, or a combination of these are prime triggers for downy mildew. The disease is most prevalent in spring and early summer, when conditions are ideal for this fungus. The disease produces spores that are spread by wind and rain. These spores can infect the leaves, shoots, and clusters of our beloved grapes.

Now what are the symptoms of mildiou or downey mildew:

• Leaf spots: The first signs often appear as small, yellow or light green spots on the upper side of the leaves. They look a bit like oil stains.
• Leaf underside: If you turn the leaf over and look at the underside, you'll see a white to light purple mold fluff. This fluff contains fungal spores.
• Leaf deformity: As the infection progresses, leaves may become distorted and curl. They may also turn yellow and drop prematurely.
• Bunches: The fungus can also affect bunches. Brown spots and fuzz can appear, and the grapes themselves can become deformed.
• Young shoots: New shoots, often the new axillary shoots, and young leaves can also be susceptible to infection. They exhibit similar symptoms, including spots, fluff, and deformities.

Symptoms can vary depending on the grape variety, the severity of the infection, and environmental conditions. It's important to regularly inspect the vineyard and take appropriate measures at the first signs of downy mildew, such as applying fungicides and implementing cultural measures to limit the spread of the fungus.

There are several strategies, both during and especially before you establish a vineyard, that can help you combat powdery mildew. Here are a few:

Choose a plot with good airflow or air circulation. Urban environments are often warmer and less windy than those outside the city. A vineyard also needs good drainage. Waterlogged areas where water often stagnates after rainfall (which is common in winter) are less suitable for viticulture.

Sloping plots usually have good water management for grapes. If your plot slopes northward, rising southward, you're getting pretty close to a perfect plot. Of course, a soil analysis should determine whether your soil is immediately suitable, needs adjustment, or is completely unsuitable.

There are proponents and opponents of varieties that aren't 100% Vitis vinifera (wine grapes). I remain neutral. I recognize the fungal resistance of PIWIs for the simple reason that in my vineyard, there are two varieties that are 100% Vitis vinifera (Chardonnay and Pinot Noir) and two varieties that are crosses with other vitis families (Johanniter and Regent). My Chardonnay and Pinot Noir suffer from fungal diseases every year, but my resistant varieties don't.

I hope it stays that way; I'm confident. It might be wishful thinking, but given that resistance to powdery mildew is genetically determined, I don't expect a change in DNA anytime soon. Will the fungi adapt? They might. But the Concorde grape (Vitis labrusca), very well-known in North America, has always been resistant to powdery mildew and will remain so. The powdery mildew fungus mutates in America as well, but the Concorde grape (Vitis labrusca) remains powdery mildew-free. Granted, the wine from the Concorde grape is undrinkable, but that grape isn't grown commercially for wine in Europe either, so a possible exception isn't ruled out.

PIWIs are therefore usually crosses of Vitis vinifera, Vitis labrusca, Vitis amurensis, Vitis riparia, Vitis rupestris , and so on. Especially clones with Vitis labrusca (e.g., Pinotin) are more resistant (though not 100% resistant) than clones with, for example, Vitis amurensis (e.g., Rondo). So choose your grape varieties wisely. Not all Vitis vinifera varieties are highly susceptible to mold, and not all PIWIs are resistant. I'm happy to help you with your choice via the contact form on this site .

Here you see a completely defoliated bunch zone during the harvest

Pruning and selecting to keep your vineyard airy

Grapevines are "growing." Not pruning is not an option. It starts in winter, when we do the soft pruning. I personally use the Guyot method, which I supplement with the Simonit method. Don't worry if you hear thunder in Cologne right now. I'll explain it later.

After the gentle pruning, we select shoots in spring (late April/early May). Buds with two shoots emerging are treated. The weakest shoot is removed. This not only improves the airiness of your foliage wall but also reduces fruit set. Excess fruit is more difficult to ripen, especially if the weather isn't favorable.

At the end of June, when the shoots reach the top of the highest wire (at 2 meters for me, but with PIWIs it can be lower, down to 1.5 meters), it's best to top off the shoots. Also, make sure to prune any branches that extend 40-50 cm beyond the row.

A final step I take is to completely or partially defoliate the bunch zone on the (north)east side of the vineyard. This allows air to circulate through the bunch zone and helps everything dry out properly in the morning or after a rain shower. When the grapes are ripening, you can also defoliate the (south)west side, but that's more to optimally ripen the fruit. Note that you should only defoliate the (south)west side once the risk of heat waves has passed. Sunburn can be detrimental to a healthy harvest.

From April until the last grape leaves my vineyard, I constantly have my two antennas on my head. I'm checking my vineyard's condition and the weather forecast for the coming period. There are weather stations connected to services that warn you about powdery mildew and downy mildew. But with my budget and the size of my vineyard, that's difficult to afford. I do have an affordable weather station that provides me with valuable information about my vineyard. It tells me how much precipitation has fallen, the general humidity, the temperatures, and the wind speed. I'll explain later why I monitor these parameters.

In your fight against fungi, it's important to monitor your plants. I look at the leaves and the bunches, if any are already present. Check the tops for white powdery mildew. Oil stains on the leaves and/or white fluff on the underside of the leaves indicate downy mildew. Some symptoms (harmless sucking traces of insects) can make you wonder if your vineyard is under attack. If you're unsure or want to preventatively check for spores on your leaves, you can perform a simple test.

• Place a few leaves in a transparent bag (freezer bag)
• Place this bag at room temperature in a dark place (in a cupboard in the living room, for example).
• Wait three days and then look at the leaves. If you don't have any symptoms of powdery mildew, you're most likely safe.

At critical points, you should apply a fungicide. Raw milk, soybean infusion, spray sulfur , copper , Luna Experience (Bayer), etc. are a few examples. They are all effective against fungi, some more potent than others. It's important to distinguish between contact fungicides and systemic fungicides. In any case, follow the manufacturer's instructions.

Contact fungicides are just as they say. They work in contact with the leaves of your vine and are even most effective when sprayed at the base of the leaf. Therefore, it's important to thoroughly reach all the leaves with your fungicide. These contact fungicides are less effective, but are readily available and can be used frequently. They also work well preventatively. However, they are usually not suitable for curative treatment. The waiting period between treatment and processing and/or consumption of the fruit is relatively short.

Systemic agents are products that are usually absorbed by the plant through the leaves and transported throughout the plant via the sap streams, where they take effect. Systemic agents are usually synthetic, chemical in nature and are produced by Bayer, BASF, and others, to name just two well-known companies. These agents are highly effective but must be applied precisely to avoid posing a risk to the winemaker and the person consuming the final wine. Some products also have a healing effect when the fungal infection is not yet too advanced. Systemic agents often have a longer shelf life because they remain in the plant longer.

A vineyard in harmony with nature, with a biological balance and biodiversity, can be a sound strategy against all diseases in the vineyard. A natural vineyard is one where nothing is added to the vineyard, but also nothing is lost. Only the juice of the grapes, which ultimately comes from rain or groundwater, disappears from the vineyard. Even the grape press residues are returned to the vineyard.

In a natural vineyard, every organism lives in harmony with each other. Underground, a network of fungi and bacteria develops, exchanging substances that make plants stronger and therefore more resilient. Contrary to what we all want, a natural vineyard develops slowly. The monoculture that is a vineyard is built up gradually, in balance with the existing flora and fauna.

A natural vineyard is a very interesting topic in itself; definitely read more about it if you're interested. Who knows, I might delve into it someday and write a comprehensive article about it.

Choosing a wine style that harvests earlier, such as sparkling wine, white wine, or rosé, can also ease your battle with powdery mildew. The less time your grapes spend on the vine, the less time they're vulnerable to powdery mildew.

But what exactly should I do in my vineyard to combat downy mildew? Well, I'll share my recipe in the chapter below:

These are photos from my first treatment in 2023. I used the 10-10-10 rule, of course (see further in this article).

A complete solution against powdery mildew isn't something you can get in a bottle; it's a process in which vigilance and making the right choices are crucial. I want to tell you right away that preventing powdery mildew (oidiüm in French and powdery mildew in English) is easier than curing it. Each season, I try to identify the key areas to consider. I'll be including both powdery and downy mildew in my action plan.

As I mentioned earlier in this article, you can apply a fungicide to kill dormant fungal spores. With a contact fungicide, you can choose your application time widely. For a systemic product, you'll need a green canopy. Otherwise, the fungicide will be nearly impossible to absorb. I apply the treatment with a spray sulfur . Don't forget this during the busy harvest and aging process of your grapes/wine.

During the winter, the vineyard is dormant, so leave it alone. At least, when it comes to fungus control. Some winegrowers apply some mulch or compost to the black strips during the winter. But as far as fungus control or pest management is concerned, it's a long vacation for the winegrower. But spring is coming soon...

What can you use to combat powdery mildew? That largely depends on the strategic choices you made before planting your vineyard. I've already discussed the location of your vineyard, your choice of varieties, choosing a biodiverse, balanced, natural vineyard, your choice of organic or synthetic products, your pruning method, and so on.

I know a winemaker with a natural vineyard who combines Piwi and Vitis vinifera varieties in his vineyard. He makes a fully organic natural wine, and he treats his vines only with raw milk. 4% raw milk, the rest water. With 100 liters of spray solution, he treats 2,000 plants.

But most vineyards can't survive on raw milk. Only because the biodiverse ecosystem in a slowly developing, natural vineyard is in balance does this winemaker succeed in protecting his vineyard from all diseases with raw milk.

There are three important preventative products in the fight against powdery mildew and downy mildew: sulfur and copper. Both are contact agents and readily available in stores.

• Sulfur
  • It is dissolved in water and applied in a protective layer to the plant, both the leaves and the grapes.
  • This layer disrupts the life cycle of mold and prevents mold spores from germinating and developing further.
  • Sulfur also has a second function. It inhibits enzymes necessary for fungal growth and development. This allows sulfur to inhibit the fungus.
• Copper
  • Has a broad spectrum of action as a contact fungicide
  • Copper also inhibits enzymes involved in the production of energy needed to form the cell wall of fungi, including powdery mildew and downy mildew.
  • Copper also has a toxic effect on fungal spores. Copper also hinders the germination and spread of fungi.
• Potassium bicarbonate or sodium bicarbonate
  • These products are sometimes colloquially called baking powder or baking soda. As the name suggests, both products contain carbon, but also oxygen and hydrogen. One product also contains potassium, the other sodium. Potassium bicarbonate is more difficult to obtain for amateur winemakers; sodium bicarbonate is readily available in the supermarket. Potassium can affect the acidity of your grapes and the resulting wine (lower acidity), and sodium, if used in excess, can be less beneficial for your soil structure. The choice is yours.

Sulfur, copper, and bicarbonate are preventative agents and must be applied according to a strict spray schedule to be effective (see further in this article). You can't start sulfur and copper treatments in the vineyard if your leaves look like white cardboard (powdery mildew) and the fluff is already thickly layered on the underside of your leaves (downy mildew). These agents also work preventatively against botrytis or gray mold.

Note: Some downy mildew is present in every vineyard. Don't panic if you see a few symptoms, especially later in the season. As long as most of your vines are unaffected, or if one plant has sufficient green, healthy foliage (more than 80% of its foliage) and the berries are largely intact, you can continue with your preventative strategy.

I spray as much as possible at the bottom of the leaf, the top is also touched of course

As a hobby winemaker, you're limited to a preventative approach. If you experience a very high mold pressure early in the year (May, June, July, etc.), you need to investigate the reason your treatment plan didn't work. On the other hand, you need to find a way to save your vineyard. You can also choose not to harvest any grapes that year and evaluate and adjust your plan for the following year. Wijnbouwweetjes is licensed to sell phytoproducts to certified winemakers who hold a phytolicense. Wijnbouwweetjes is also licensed to apply phytoproducts to those who don't. If you're really struggling, please contact us. Due to time and distance constraints, we can't always help everyone, but we'll do our best.

Synthetic or chemical fungicides to combat powdery mildew and downy mildew are available in many forms and formulations, and are produced by many different manufacturers. As mentioned, they often have a systemic effect and are absorbed into the plant's sap streams to exert their effects. Therefore, these products are often more effective and last longer than sulfur , copper , and bicarbonate. These products do have some drawbacks, however. They can also kill beneficial organisms that are beneficial to your plants and ecosystem, and they also require a waiting period between the time of treatment and harvest (processing) of your grapes. A product with a 28-day waiting period should not be applied 10 days before the expected harvest.

When do you start preventatively protecting your vineyard against powdery mildew? Well, there are three indications that you can start. If two of the three conditions are met, I often fill my backpack sprayer for the first time.

• When average daily temperatures rise around 10°C.
• If there are 10 leaves on your shoots (including folded leaves)
• The moment when 10 mm of rain has fallen per m²

That number 10 is very important. Average daily temperatures are calculated by adding the nighttime temperature and the daytime temperature together. For example, 15°C during the day and 5°C at night is an indication that your mission is about to begin. You can count 10 leaves, but even folded leaves count. If the shoots have grown out by approximately 15 cm or more, you won't be far off. On my weather station, which includes a rain gauge, I look at the minimum/maximum temperatures, but also at the number of mm of rain falling. In practice, the first treatment often falls at the end of October.

Personally, I always use sulfur or bicarbonate of soda. Never both, always one or the other. I usually use sulfur from the beginning of the season until the grapes turn brown (véraison). After véraison, I use sodium bicarbonate. This is to introduce sulfur into my juice/must, which can hinder fermentation. I actually always use copper . Perhaps, when there's less downy mildew pressure, you don't always need to add it, but I don't like to risk downy mildew developing.

From the 10-10-10 starting signal, I apply preventative treatment every 15mm of rain, or at least every three weeks until the end of June. Let me explain. It's logical to rinse the contact fungicides off your leaves every 15mm of rain, so you need to reapply. Spray as much as possible on the underside of the leaves.

But if it doesn't rain, I still treat every 21 days until the end of June. That's about three treatments. I do these treatments because your plant experiences a significant amount of vegetative growth during that period. A new leaf is an unsprayed leaf that is then unprotected. By the end of June, the strongest vegetative phase is over, and I rely on my 15mm of rainfall.

There are two other crucial moments when you should definitely give treatment in the cluster zone:

I always spray after the blooming period (flowering). It's between the blooming period and the fruiting period (nounaison). This is when the grape bulbs themselves are first accessible, but also unprotected. So, regardless of the 15mm rain rule and the 21-day rule, I apply a treatment when the leaves of the mini flowers on the future bunches fall off. If you tap the shoot and see leaves falling, it's best to apply a treatment at that time.

The last time it's essential to treat is when the bunches are closing, between fruit set and veraison. This is the last time you can spray a protective layer around the berries.

I also use Curagrow foliar fertilizer in my spray mix, which adds extra magnesium from kieserite. It also reduces the hygrometry on the sprayed surfaces, the humidity. This creates a very unfavorable environment for mold.

From the 10-10-10 starting shot until the end of flowering (values ​​per 10 liters of water) +/- end of May:

• Sulphur: 80ml Sulfis spray sulphur per 10 litres
• Copper: 40 grams (4 sachets) Cuprex anti-fungal porridge (42 days waiting period)
• Curagrow foliar nutrition: 30 ml

• Sulphur: 80ml Sulfis spray sulphur per 10 litres
• Copper: 40 grams (4 sachets) Cuprex anti-fungal porridge (42 days waiting period)
• Curagrow foliar nutrition: 30 ml

• Sodium bicarbonate: 50 grams (no waiting period)
• Copper: 20 grams (2 sachets) BSI Ecopur copper fungicide (no waiting period, ideal for the last weeks before harvest in case of mold pressure)
• Curagrow : 30 ml

All the above formulations contain sufficient wetting agent. This promotes wetting and adhesion to the surface.

As mentioned, we spray every 15mm of rain, every 21 days before the end of June, and during flowering. It's best to combine as many conditions as possible in a single spray. If it rained 12mm at the end of May, your last spray was 16 days ago, or the flowering phase is ending (paddles or leaves fall off when you tap the stem), combine these factors in a single spray. Then wait another 21 days or 15mm of rain.

Check your plants regularly; don't blindly follow the spray schedule. If you're unsure, test with a few leaves in a plastic bag (place them higher). Feel free to send me a photo of your leaves or vines. I'm happy to help.

Powdery mildew and downy mildew in our vineyard can seriously spoil the fun, literally. So be vigilant, stick to the spraying schedule, and don't panic too quickly.

I wish you a mold-free vineyard.

Best regards, Tom