For organic vintners, copper-based sprays have proven to be the most effective tool for combatting downy mildew. Prevalent in regions with warm, humid springs and summers, the fungal disease can wipe out entire vintages. Left untreated, it can even kill grapevines. For organic producers in large parts of northern Europe and the eastern U.S., copper is therefore not just a tool, but a necessity.

In January 2019, the European Commission enacted a new law lowering the amount of copper that could be used in organic farming. The ruling came after the European Food Safety Authority published a high-profile risk assessment, which showed that copper compounds pose risks for farm workers, animals, and vineyards.

In regions such as Bordeaux, organic producers must now adapt to these new limits while still effectively combating downy mildew. Otherwise, the only options are switching to synthetic chemicals or accepting significant and costly yield reductions. There may be some hope on the horizon, though: Numerous trials around the world are testing whether new, organically acceptable fungicides could supplement or even one day replace copper.

To find out how organic vintners are coping with the new limits, and to see what the future holds for regions susceptible to downy mildew, VinePair spoke with multiple Bordeaux producers and viticultural scientists with knowledge of potential innovations.

The Pros and Cons of Using Copper

While downy mildew is by no means the only challenging fungal disease vintners face, it is among the most significant. “If left unchecked, it has the potential to kill your vine [because] it reduces [the vine’s] ability to store the nutrients it needs to get through winter,” explains Dr. Katie Gold, assistant professor of grape disease ecology and epidemiology at Cornell University. ”Many of the other fungal diseases will impact crops within the season, but they won’t necessarily kill the vine.”

Since the 1800s, vintners have combated downy mildew with a copper solution called bouillie bordelaise. Otherwise known as Bordeaux mixture, the solution contains a mix of copper sulfate, lime, and water. While allowed in organic viticulture, prolonged use of high concentrations of Bordeaux mixture (and other copper-based solutions) can be extremely harmful to vineyards.

“In Europe, where they use it extensively, copper can leave heavy metals on the surface of the soil, which can not be metabolized by microorganisms,” says Dr. Akif Eskalen, plant pathologist and extension specialist at UC Davis.

When copper accumulates, it affects the concentration of useful microorganisms, the pH of the soil, and ultimately grapevine growth, Eskalen explains. In regions like Bordeaux, where copper solutions have been sprayed for over a century, this can be a common problem. The issue is further compounded by non-organic producers who use copper alongside chemical herbicides, a combination that can be toxic for soils.

But elsewhere in the world, in regions where downy mildew is not as common, copper accumulation is not an issue.

In 2019, grower-led organization Organic Winegrowers New Zealand (OWNZ) commissioned a soil assessment study, measuring the level of copper in soils after eight years of organic management. “Our independent researchers found that there was no sign of elevated copper levels in the organic vineyard soils after eight years of organic management,” says Rebecca Reider, OWNZ’s national coordinator. “We weren’t surprised by this result, but we were happy to receive it.”

How Producers Are Adapting in Regions With High Downy Pressure

This year marked the second vintage that European organic producers have been adapting to the new copper limits. Previously, vintners could spray up to 6 kilograms per hectare per year (around 5 pounds per acre). Now, they can apply 4 kilograms per hectare per year (around 3.5 pounds per acre). The law does allow levels to be taken over a seven-year average, meaning that in particularly wet vintages, producers can exceed that limit.

This year has proven to be one such vintage in Bordeaux.

Pierre Cazeneuve, owner of Château Paloumey in the Médoc, describes 2020 as one “one of the most difficult mildew pressures” he’s seen in the last 10 or 15 years. Cazeneuve has been practicing organic farming for five years, and when the copper reductions were first announced, a vintage such as this was his worst nightmare. But with just one month remaining where he will potentially have to spray, he’s surprised to find that he has so far only used half of his yearly allowance.

How has he adapted? Firstly, Cazeneuve dropped the concentration of copper he’s including in his sprays by around 25 percent. “Regularity and accuracy of spraying seem to be the best solution rather than quantity,” he says. Cazeneuve also started essential vineyard work as early as possible, and has been vigilant in keeping on top of processes that improve aeration. He’s also kept a close eye on the weather, tracking it with four different cell phone apps to ensure he’s spraying at least a day in advance when rain is on the horizon, though this is nothing new. “Some people look at Twitter, I look at weather forecasts,” he says.

While his new regime has increased his workload significantly, Cazeneuve now believes the lower copper limits seem feasible. But he stresses that it will take a few more years and statistical data from around Bordeaux to prove whether that is the case for all organic producers.

Others need less convincing. Matthieu David-Beaulieu, winemaker at St. Emillion’s Château Coutet, agrees that it’s been a particularly trying vintage. But at no point has he worried about exceeding the maximum copper limits.

Château Coutet has been certified organic since 2012, and practiced organic farming for many years before that. David-Beaulieu says the experience built up in this time, as well as the health of his vineyards, have proven effective during this challenging vintage. But for those who recently switched to organic farming, it’s been a process of learning. “They want to use a lot [of copper] to protect [the vines], they are not used to spraying such small amounts,” he says.

Copper Alternatives on the Horizon?

For producers wishing to reduce their copper use, there are currently only a handful of commercially available organic products. Examples include EF 400, which contains a mix of essential oils, and Certis USA’s OSO. But these products work only as a supplement, and are not a full-blown replacement for copper.

Around the world, trials are taking place to test the efficacy of other solutions. In Italy, some success has been found using chitosan, a naturally occurring polymer that’s extracted from the outer shells of crab and other shellfish. Laminarin, a carbohydrate extracted from brown algae, also appears to be a potentially effective supplement.

Recent studies using a microorganism called bacillus subtilis have also proven effective at inhibiting the growth of fungi such as downy mildew. But the success of biocontrol agents such as this is limited by climate, weather conditions, and location of plants, says U.C. Davis’s Eskalen.

“I’ve done a lot of research with new products, but the main problem is efficiency,” says Etienne Laveau, a Bordeaux-based consultant who specializes in organic viticulture. Laveau says that, at best, the products he’s tried have been 40 percent as efficient as copper — and that was in vintages that were much less challenging than 2020.

Laveau agrees that alternatives and supplements need to be found. But for now, he says, there remains only one path forward: “We hope that we can continue to use copper for a long time — maybe with other products to help us, but without copper it would be impossible.”