Mildew and downy mildew on grapes in the vineyard

Mildew (also called powdery mildew) and downy mildew on grapes, the fear of many wine growers. In this article I will teach you about, recognize, prevent and combat the fungal diseases. I hope that after reading this article you will have the strategy and weapons to keep your vineyard free from a high infection of (downy) mildew.

Harvesting healthy, ripe and intact grapes in the fall is so important to make a beautiful, flavorful wine. Unhealthy, damaged and unripe grapes are hell to vinify. With the information tips and my prevention plan from this article you should be able to control mildew. I wish you lots of success.

The main difference between powdery mildew and downy mildew is that powdery mildew looks like spilled powder on the leaves, while downy mildew causes the leaves to droop and form yellow and brown oily spots. Moreover, the spores of powdery mildew appear on both sides of the leaves. But the spores of downy mildew are only on the underside of the leaf. Moreover, powdery mildew produces a thick web of white spores that cover the leaves, stems, and shoots, while in downy mildew the 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. In general, powdery mildew is an ascomycete fungus (family sac fungi), while downy mildew is an oomycete fungus (a water mold). So 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 throughout the year.

Every year, powdery mildew strikes our vineyards, but also cucumber plants, lettuce, courgettes, nut trees and dozens of other crops. The fungus overwinters in the vineyard in the old wood. You can see it from the yellow or black spots on the wood from the previous year. That is why in the autumn, after the harvest, a treatment with a fungicide is recommended in the vineyard (you will find my composition later in this article). You win one point in your battle against the fungus before the next season starts, but the game is far from won.

Mildew thrives in warm and dry conditions. Despite the fact that the fungus has the word 'dew' in its name, it likes dryness. Then it can spread well through your vineyard. Once it gets humid, the white powdery layer appears on your leaves and also on the berries of your vine. Between 15 and 27 degrees, the pest has a great time. The white sheen on your leaves prevents photosynthesis and the white sheen on your berries damages the cell structure of your berries with all the consequences when ripening. Berries can burst open, attract wasps and provide a home for unwanted bacteria.

Downy mildew is caused by the fungus Plasmopara viticola. It is unfortunately a 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 real instigators for downy mildew. The disease is most prevalent in spring and early summer, when conditions for this fungus are ideal. The disease produces spores that are spread by wind and rain. These spores can infect the leaves, shoots and bunches of our beloved grapes.

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. It looks a bit like oil stains
• Underside of the leaf: If you turn the leaf over and look at the underside, you can see a white to light purple mold fluff. This fluff contains spores of the mold.
• Leaf deformity: As the infection progresses, leaves may become deformed and curl. They may also turn yellow and drop prematurely.
• Bunches: The fungus can also affect bunches. Here brown spots and fluff can appear, 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 show similar symptoms, including spots, fluff and deformities.

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

There are a number of strategies during but especially before you plant a vineyard that can help you in the fight against mildew. I will list them:

Choose a plot with good airflow or air circulation. In an urban environment it is often warmer and less windy than outside the city. A vineyard also needs good drainage. Marshy areas where water often remains after rainfall (you see this more often in the winter) are less suitable for viticulture.

Plots that slope down usually have good water management for wine grapes. If your plot then slopes down towards the North, so climbs towards the South, you come pretty close to a perfect plot. Of course, a soil analysis must show whether your soil is immediately suitable, needs to be adjusted or is not suitable at all.

There are proponents and opponents of the varieties that are not 100% vitis vinifera (wine grape). I myself remain neutral. I recognize the fungal resistance of PIWIs for the simple reason that in my vineyard there are 2 varieties that are 100% vitis vinifera (Chardonnay and Pinot noir) and 2 varieties that are crosses with other vitis families (Johanniter and Regent). My Chardonnay and Pinot noir suffer from fungal diseases every year, my resistant varieties do not.

I hope it stays that way, I trust that. It could also be wishful thinking but the fact that resistance to mildew is genetically determined, I do not expect a change in DNA in the short term. Or will the fungi adapt? That is possible. But the 'Concorde' grape (vitis labrusca), very well known in North America, has always been resistant to mildew and will remain so. The mildew fungus also mutates in America but the Concorde grape (vitis Labrusca) remains mildew-free. Agreed, the wine from the Concorde grape is undrinkable but that grape is also not grown commercially for wine in Europe, a possible exception not excluded.

The PIWIs are therefore in most cases crosses of vitis vinifera, vitis labrusca, vitis amurensis, vitis riparia, vitis rupestris , ... Especially the clones with vitis labrusca (for example Pinotin) are more resistant (read not 100% resistant) than clones with vitis amurensis (such as Rondo) for example. So choose your grape varieties wisely. Not all vitis vinifera varieties are very sensitive to mold, not all PIWIs are resistant. I am happy to help you with your choice via the contact form on this site .

Here you see a completely defoliated bunch zone during picking

Pruning and selecting to keep your vineyard airy

Grapevines are 'growers'. Not pruning is not an option. It starts during the winter when we do the soft pruning. I myself use the Guyot method in which I use the Simonit method. Don't worry if you hear it thundering in Cologne now. I'll explain it later.

After the soft pruning, we will select shoots in the spring (end of April/beginning of May). Eyes where two shoots emerge are tackled. The weakest shoot is removed. This not only benefits the airiness of your foliage wall but also ensures less fruit. Too much fruit is more difficult to ripen, especially if the weather is bad.

At the end of June, when the shoots are at the top of the highest wire (at 2 meters for me, with PIWI's this can also be lower to 1.5 meters), it is best to top the shoots. You also make sure that the branches that come 40-50 cm out of the row are pruned.

A final action I do is that I completely or partially defoliate the bunch zone on the (north)east side of the vineyard. This allows air to pass through the bunch zone and everything dries out well in the morning or after a rain shower. When the grapes are ripening, you can also defoliate the (south)west side, but that is more to optimally ripen your fruit. Note that you only defoliate the (south)west side when the risk of heat waves has passed. Sunburn can be detrimental to a healthy harvest.

Between April and until the last grape has left my vineyard, I constantly have my two feelers on my head. What does my vineyard look like and what are the weather forecasts for the coming period. There are weather stations that are connected to services that warn you about mildew and downy mildew. But with my budget and the size of my vineyard, that is difficult to afford. I do have an affordable weather station that gives me valuable information about my vineyard. It gives me the precipitation that has fallen, the general humidity, the temperatures and the wind force. Later I will explain why I monitor these parameters.

It is important in your fight against fungi to look at your plants. I look at the leaves and the bunches if they are already there. Check the top for white powdery mold (powdery mildew). Oil stains on the leaves and/or white fluff at the bottom of the leaves indicate an attack by downy mildew. Some symptoms (harmless sucking traces of insects) can make you doubt whether your vineyard is 'under attack'. If you are in doubt or want to know preventively whether there are traces on your leaves, you can do 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 have no symptoms of (powdery) mildew then you are most likely safe

At critical points you should apply a fungicide. Raw milk, soybean infusion, spray sulphur , copper , Luna Experience (Bayer), ... are some examples. They all work against fungi, one more strongly than the other. It is important to make a distinction between contact fungicides and systemic fungicides. In any case, follow the instructions of the manufacturer.

Contact fungicides are as they say. They work in contact with the leaves of your vine and are even most effective when sprayed at the bottom of the leaves. It is therefore important that you reach all the leaves well with your fungicide. These contact agents are less efficient but are freely available and you can use them often. They also work well preventively. You usually cannot treat them curatively (healing). The waiting time between treatment and processing and/or consumption of the fruit is quite short.

Systemic agents are products that are usually absorbed by the plant via the leaves and are transported further over the plant via the sap streams and do their work there. Systemic agents are usually of a synthetic, chemical nature and are produced by Bayer, BASF, ... to name two well-known companies. These agents are very efficient but must be applied very correctly in order not to pose a danger to the winegrower himself and the person who consumes 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 waiting period because they remain in the plant longer.

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

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

A natural vineyard is a very interesting subject in itself, definitely read more about it if you want to know more about it. Who knows, I might delve into it myself one day and write an extensive article about it.

If you choose a wine style that harvests earlier, such as sparkling, white or rosé, this can also ease your battle with powdery mildew. The less time your grapes spend on the vine, the less time your grapes can fall victim to powdery mildew.

But what should I do now in my vineyard against (false) mildew? Well I will reveal my recipe in the chapter below:

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

A total solution against oidiüm is not in a bottle, the solution is a process in which being alert 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. I try to map out the points of attention per season. I will include both powdery mildew and downy mildew in my action plan.

As I wrote earlier in this article ... you can do a follow-up treatment in the vineyard with a fungicide to put a stop to the dormant fungal spores. With a contact fungicide you can choose your moment widely. Let's say before mid-November. With a systemic agent you still need a green foliage wall. Otherwise the fungicide can hardly be absorbed. I do the follow-up treatment with spray sulfur . Don't forget this in all the hustle and bustle of the harvest processing and education of your grapes/wine.

During the winter the vineyard sleeps, leave it alone. In terms of mold control at least. There are wine growers who still apply some mulch or compost in the black strips in the winter. But as far as mold control or control is concerned, it is a big vacation for the wine grower. But spring is coming soon ..

What can you use against mildew? That depends largely on the strategic choices you made before planting your vineyard. I already told you about the location of your vineyard, the choice of your varieties, the choice for a biodiverse balanced natural vineyard, your choice for organic products or synthetic products, your pruning method, ...

I know a wine grower with a natural vineyard, who combines piwi varieties and vitis vinifera varieties in the vineyard, who makes a completely organic natural wine and he treats his vines only with raw milk. 4% raw milk, the rest water. With 100 liters of spray liquid he treats 2000 plants.

But most vineyards do not survive on raw milk. Only because the biodiverse ecosystem in a slowly built up natural vineyard is in balance, this wine grower succeeds in protecting his vineyard with raw milk from all diseases.

There are three important preventative products in the fight against powdery mildew and downy mildew. These are sulphur and copper. Both contact agents and freely available in the trade.

• Sulfur
  • 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.
  • In addition, sulfur has a second function. Sulfur inhibits enzymes that are necessary for the growth and development of fungi. 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 the spores of fungi. Copper also hinders the germination and spread of fungi.
• Potassium bicarbonate or sodium bicarbonate
  • These products are sometimes called baking powder or baking soda. As the name suggests, both products contain carbon, but also oxygen and hydrogen. One product also contains potassium and the other sodium. Potassium bicarbonate is more difficult to obtain as a hobby wine grower, sodium bicarbonate can be found in the supermarket. Potassium can have an effect on the acids in your grapes and final wine (less acids), sodium can be less beneficial for your soil structure if used excessively. The choice is yours.

Sulphur, copper and bicarbonate are preventatives and must be applied in a strict spray schedule to be effective (see further in this article). You cannot start sulphur 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 products also work preventatively against botrytis or grey rot.

Please note: a little (false) mildew is present in every vineyard. Do not panic if you see a few symptoms, especially later in the season. As long as the majority of your vines are not affected or when one plant has enough green, healthy leaves (more than 80% of your foliage) and the berries are also largely intact, you can continue with your preventive strategy.

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

As a hobby wine grower you are limited to the preventive approach. If you have a very high mold pressure early in the year (May, June, July, ...), then on the one hand you have to look for the reason that your treatment schedule did not work. On the other hand you have to find a way to save your vineyard. You can also choose not to harvest grapes that year and evaluate your plan of action and adjust it for the following year. Wijnbouwweetjes has a license to sell phyto products to people who are certified for this, wine growers who have a phyto license. Wijnbouwweetjes also has a license to apply phyto products to people who do not have a phyto license. If you are really in trouble, contact us. Due to time and distance we cannot always help everyone, but we do our best.

Synthetic or chemical fungicides to combat mildew and downy mildew come in many forms and compositions and by many manufacturers. As mentioned earlier, they often have a systemic effect and are absorbed into the sap flows of the plant to perform their function there. That is why these agents often work more effectively and longer than sulphur , copper and bicarbonate. These agents also have some disadvantages. They can also kill beneficial organisms that are good for your plants and ecosystem and they also have a waiting period between the moment of treatment and harvest (processing) of your grapes. An agent with a waiting period of 28 days may not be applied 10 days before the expected harvest.

When do you start to protect your vineyard preventively against mildew? Well, there are 3 indications that you can start. If 2 of the 3 conditions are met, I often fill my backpack sprayer for the first time.

• When average daily temperatures rise to 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 day temperatures are calculated by adding the night temperature and the day temperature together. For example, 15°C during the day and 5°C at night is an indication that your mission is about to start. You can count 10 leaves, but note that folded leaves also count. If the shoots have grown out +/- 15 cm or more, you will not be far off. On my weather station with, among other things, a rain gauge, I look at the min max temperatures but also at the number of mm of rain that falls. In practice, the first treatment often falls at the end of October.

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

From the 10-10-10 starting shot I treat preventively every time there has been 15mm of rain or until the end of June at least every 3 weeks. Let me explain myself. Every 15mm of rain it is logical to rinse the contact fungicides from your leaf so you have to apply a new layer. !! Spray as much as possible on the underside of the leaf!!

But if it doesn't rain, I still treat every 21 days until the end of June. That's about 3 treatments. I do those treatments because there is a very large vegetative growth of your plant in that period. And a new leaf is an unsprayed leaf that is then unprotected. At the end of June, the strongest vegetative phase is over and I fall back on my 15mm rainfall.

Another 2 crucial moments when you should definitely give treatment in the cluster zone:

A moment that I always give a spraying is after the floraison (flowering). Between the floraison and the nouaison (fruiting). Then the grape bulbs themselves are on the one hand accessible for the first time and on the other hand they are also unprotected. So regardless of the 15mm rain rule and the 21 day rule I do a treatment when the leaves of the mini flowers on the future bunches fall off. If you tap the shoot and you see that leaves fall down, you should do a treatment at that moment.

The last time it is a must to treat is at the closing of the bunches between fruit set and veraison. This is the last time you can spray a protective layer around the berries.

In my spray mix I also use a foliar feed Curagrow which adds extra magnesium from kieserite. It also lowers 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 litres 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 bags) 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 compositions contain sufficient spreading agent. This promotes spreading over and adhesion to the sheet.

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

Check your plants regularly, don't blindly follow the spray schedule. Do the test with a few leaves in a plastic bag (them higher) if you have doubts. Feel free to send me a picture of your leaves or grapes. I'll be happy to help you.

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

I wish you a mold-free vineyard.

Best regards, Tom