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.

Powdery mildew is a common fungal disease that can affect not only grapes but also numerous other plants. In vineyards, we distinguish between two types: powdery mildew and downy mildew. Powdery mildew is caused by the fungus Uncinula necator and is known for the white, powdery coating that forms on leaves and stems. Downy mildew, on the other hand, is caused by Plasmopara viticola and manifests differently.

Both fungi can cause significant damage to plants, especially if not identified and treated promptly. It's therefore important to know which fungus you're dealing with so you can choose the right treatment and minimize the impact on your grapes and the final flavor of your wine.

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). Both fungi feed on plant tissue, leading to wilting and death of parts of the plant. 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.

Downy Mildew - The First Symptoms

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.

Downy mildew on the bottom of the leaf

Downy mildew also causes fluff on the berries

After the fluff, the berries dry out due to downy mildew

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.

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

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

After the gentle pruning, we select the new shoots in spring (late April/early May). Buds where two shoots emerge from the fruiting pod are targeted. The weakest shoot is removed. This not only improves the airiness of your foliage wall but also reduces fruit production. Excess fruit is more difficult to ripen, especially if the weather is unfavorable.

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 (shorten) 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 before flowering. This allows air to circulate through the bunch zone and allows everything to dry out properly in the morning or after a rain shower. When the grapes ripen in August/September/October, you can also defoliate the (south)west side, but this is mainly to optimally ripen the fruit. Note that you should only defoliate the (south)west side after the risk of heat waves has passed. Sunburn can be detrimental to a healthy harvest.

You'll notice this option isn't listed first. Try to follow the previous tips carefully, as fungicides are beneficial for the wild yeasts and yeast nutrients present on your grape skins.

At critical points, you should apply a fungicide. Raw milk (4 to 8% solution in water), soybean infusion, spray sulfur , copper , Luna Experience (Bayer), etc. are some examples. They are all effective against fungi, some more powerful than others. It's important to distinguish between contact fungicides and systemic fungicides. In any case, follow the manufacturer's instructions for use.

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. Examples of contact fungicides are sulfur and copper.

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.