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Prof. Dr Randolf Kauer is a professor of organic viticulture at Geisenheim University and an organic winegrower. He also researches fungal infections such as peronospora, which is currently causing massive damage in France and Italy. Kauer reports on the limits of plant protection and new opportunities for organic winegrowers.

Prof. Dr Randolf Kauer
Steffen Böttcher

"Winegrowers in Italy fear crop losses of 40 percent", "Hotline set up for "traumatised" Bordeaux winegrowers", "Six percent of organic winegrowers in France want to give up certification": these are headlines of the past weeks, triggered by the downy mildew (Plasmopara viticola or Peronospora) infestation, which has reached "unprecedented levels" in some areas due to the persistently wet weather conditions.

"It was only a matter of time before something like this happened, because weather extremes are increasing. Under these conditions, the power of vine protection is then also limited. Yield losses are then unavoidable, and this also makes itself felt economically," is how Prof. Randolf Kauer from Geisenheim University describes the threat situation.

In cooler growing zones such as Germany or Austria, such weather conditions have occurred again and again in the past decades, so vintners there have learned to deal with downy mildew better. The scientist and organic vintner recalls the 2016 and 2021 vintages in Germany. In 2021, he himself experienced in a test facility in Geisenheim in the Rheingau that the Merlot, which is now also particularly affected in Bordeaux, was carried off while still in the late phase of berry development. "The grapes were already the size of peas and we thought we had the plant clean and everything under control. Then the peronospora rushed in with high infection pressure and in a few days the harvest was destroyed." With Merlot, the grapes are extremely sensitive, more so than the leaves. That is what causes the great damage.

As soon as pea-sized berries are infected, they can hardly be saved.

Geisenheim University

Winegrowers in "Plasmopara panic

However, Kauer does not accuse the affected winegrowers in France and Italy of neglecting plant protection. With such extremely high infection pressure, all winegrowers would reach their limits, regardless of whether they work organically or conventionally: "We had the same thing in Germany in 2016 and 2021. In 2016, the infections began shortly before flowering and lasted for four weeks. This is particularly dangerous because the plants are extremely sensitive at this stage. Whole bunches of grapes were also destroyed very quickly. And when you think the berries are safe, there is still great danger because the fungus can penetrate through the stems. When it rains a lot, it is no longer possible to protect the plants because the tractors can no longer drive over the soggy soil.

The critical factor is time

Even if it is still possible to drive over the vineyards, organic farms need a correspondingly high level of application technology to be able to treat their areas in a maximum of two days, if possible before the next infection. Conventionally working winegrowers with systemically acting agents could extend the treatment intervals, but: "The timing and the application duration are the critical factors".

With sufficient clout in application technology, he says, one must concentrate on the biology of the pest and try not only to react, but to work preventively. "Peronospora we have had in Europe since the end of the 19th century. Like phylloxera and powdery mildew, it was introduced from overseas. Since 1885, plant protection has been a necessary evil - and a compulsory programme. Winegrowers, especially organic winegrowers, have gained a lot of experience in the meantime. The biology of the pathogen has been studied in great detail and we understand how the fungus develops under certain conditions. After that, action has to be taken." Unfortunately, weather forecasts are only reliable for a maximum of three days in advance. It is almost impossible to assess the weather situation correctly. Therefore, vine protection is always preventive.

Every "oil spot" produces a myriad of spores

Geisenheim University

The most important and most effective means in organic viticulture is still copper. In most years, the permitted three kilos per hectare and year are sufficient in Germany. In extreme years like 2016 and 2021, the limit was raised to four kilogrammes. Kauer calculates how quickly the limits of copper application are reached with continued rainy weather and warmth: "In the initial phase of the infection, you can get by with smaller amounts, you use 100 to 200 grams per hectare. But when it rains like it did this year, it washes off again and the coating has to be renewed continuously. Under these extreme conditions, it makes no sense to apply less than 300 grams per spray. After twelve to 15 passes, the upper limit of three kilograms is reached. With an application rhythm of four to seven days in disaster years, that doesn't take two months."

Even if an organic vintner does everything right and has his plant protection under control, the spores spreading en masse from neighbouring vineyards can still ruin everything, Kauer explains: "Every berry that is infested is lost. Even individual berries that may still be healthy don't do much good, because usually the foliage is also largely destroyed and the quality of the remaining crop is poor."

Potassium phosphonate as an opportunity

Yield losses are almost unavoidable in case of infestation

Geisenheim University

For Kauer, the active ingredient potassium phosphonate is a great hope in the fight against peronospora, especially for organic farms. However, this is currently not approved in the EU organic regulation, mainly because representatives from France, Italy and Spain have so far categorically rejected it. "I have always advocated that we need an additional active ingredient besides copper in times of crisis. The organic associations are currently preparing a dossier together with the German government that is intended to stimulate this discussion anew in Brussels. We have to be honest enough to say: In such extreme years as 2023, we are not sufficiently capable of acting with the current copper quantities. We have gained a lot of experience with phosphonate in Germany when it was still used as a tonic. We will not be able to replace copper with it, but we should be able to use these two active substances in combination. This would enable us to harden off the vine decisively, especially in the sensitive period until the end of flowering. And hardening off is what organic viticulture is all about: strengthening the vine first. This way, organic winegrowers could at least get off lightly even in emergency situations like 2023."

At the moment, the representatives from Germany, Austria and the Czech Republic are the only ones calling for organic winegrowing at the EU level. Yet copper is a candidate for substitution in EU plant protection law, and an increase in quantities is hardly up for debate. It is still necessary to use it, but according to Kauer, the discussion about the authorisation of potassium phosphonate is all the more important.

Whether the current situation could soften the rejection? "I hope so. But I am sorry that something like this has to happen first. The French, Italians and Spaniards have rejected it because they did not know such an infestation pressure before. But we have to face reality, which is that the weather changes. If we don't develop crop protection further, we will increasingly be defenceless against it."

New opportunities for control in organic farming

But experiments are also being conducted with new ways of applying copper. Copper cans ("CuCaps") encapsulated in fat coatings continuously release the active ingredient. Extracts from grapevine and liquorice show promising results. UV irradiation could complement chemical crop protection, but probably not replace it, especially in conditions like 2023.

And finally, according to Kauer, Piwi vines can save 70 to 80 per cent of plant protection and in some years do without copper altogether. Nevertheless, he continues to plead for research on phosphonate. "We have known this for a long time and its mode of action is well known because it is used in different plant crops. It is toxicologically safe, and at some point we have to allow at least a bridging technology that will keep us able to act for the years to come." He sums up: "In northern Central Europe, we already learned in the 1980s how to cope with small amounts of copper against downy mildew, even if we did not always keep it under control. Now, climatic extremes are also presenting us with crucial challenges to further develop organic viticulture. Here, too, the discussion about sustainability increases the pressure on producers to engage in conversion. However, it is all the more important to use this successfully and with low risk. If you as a winegrower have no yield, you will be broke at some point, and then the economic sustainability of the system will come to an end. We have to learn our lessons from this, so I hope for a constructive discussion at European level."

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