Protecting your wine harvest with essential oil

When Chris Pelzer saw his vines inundated in 2021 after the floods in the south of Limburg and then saw his entire harvest fail due to a disease, he decided to explore how he could prevent such disasters in future. He knocked on the door of his Alta Mater, Radboud University, and joined biologists Janny Peters and Rob de Graaf to find a biological approach to nip downy mildew in the bud.

Pelzer was always interested in plants, especially in combination with microbes. ‘If you can get microbes and plants to work together, they can handle a lot more stress than when you tinker with a plant genetically,’ says the wine grower. After studying biology at Radboud University, Pelzer started to work full-time at his father's organic vineyard, ‘Domain Aldenborgh’. His interest in helping plants emerged (quite literally) in 2021. Over the course of two days, Chris' vines were flooded with 190 litres of water per m2. ‘It was catastrophic. I sank with my boot right into the ground on our property, whereas before the flood, it had all been solid ground.’ Within three weeks, there was not a leaf left on any of the vines. It was the ideal breeding ground for downy mildew – a vicious pathogenic that makes grapes shrivel and causes entire stems to burst open.

Blue from the copper

Downy mildew is a well-known problem among wine growers. It can be effectively controlled organically, using copper. ‘But that does mean you're working with heavy metals,’ Pelzer says. ‘These metals accumulate in the soil. Some vineyards turn blue from the copper.’
You can also use compost tea, but that is only effective locally. ‘If you only spray half a leaf, only that half will be protected. And when it rains, it all washes away.’

Local solutions are not reliable enough, which means you need something systemic, as Chris remembered from his studies. An agent that goes into the plant, or better yet, that prompts the plant to fight diseases itself. ‘That means making a whole plant resilient, so that it is strong enough to repel the downy mildew,’ Radboud University researcher Janny Peters explains. Pelzer adds: ‘Fungi have a cell wall of chitin, a kind of shrimp-like skin. Plants can produce chitinase, an enzyme that breaks down chitin. If downy mildew tries to enter a plant, it will have a problem, because the plant will be able to break down its cell wall. But a vine sometimes needs some help with that.’


With this idea in mind, Pelzer contacted Janny Peters and Maastricht University. He had read a study in which positive results were achieved with oregano oil. Peters: ‘It was already known that essential oils can help plants and can be used as a bio-pesticide. But we were looking for a systemic reaction in the plant, so that the plant could solve the problem itself. Otherwise, you have to keep spraying with oil, which takes a lot of time and money. Moreover, oil is sensitive to external factors, such as rain. We needed a method that was stable.’ The biologist wrote an IRP voucher for an interdisciplinary research study, and got it: she involved chemists from Radboud University to ‘package’ the oil and investigate how to achieve slow release in the plant's environment. Maastricht University provided M4i, a technology that makes it possible to see exactly which substances are on a leaf and where.

24-hour vapour

Peters put students to work and hired junior researcher Julia Mars. Mars is now conducting the research in the Radboud University lab: “The plants are exposed to oregano oil vapour for 24 hours, and are then infected with downy mildew. Ten days later, we check how the plants look. We deliberately separate the moments of exposure to the oil and the mildew, because in that way, we can really see whether the reaction is systemic. If you add the oil and the mildew at the same time, the mildew may be repelled by the oil and not by the plant.’

The data are still being collected, but so far, the results are very striking. ‘We see that plants that have had the treatment are a lot less infected or sometimes not infected at all, while plants without treatment are full of mildew.’

Rolling eyes

Pelzer sees the same thing in his vineyard: the plants that are treated with oregano oil prove to be strong and resilient. And this hands-on experience in turn benefits Mars and her work in the lab. Mars sees many advantages in working with Pelzer: ‘Working with Chris provides interesting insights. We hear how things go in practice, and what he would like to see happen. That is not something we usually think a lot about when we work in the lab.’

Working with essential oils makes sense to the researchers, but sometimes less so to outsiders. Mars: ‘When you tell people that you're doing research on essential oils, you can notice that they're biased against it. But essential oils are substances made inside a plant. It's just chemistry.’ Peters adds: ‘People sometimes roll their eyes when we talk about it, but we really believe there’s something to it. We also see it in our lab. And Chris sees it in the field.’ The wine grower's plants are struggling at the moment, mainly due to the drought. Pelzer: ‘That is why it is extra important to make the plants as resilient as possible. If we can use essential oil to make our plants stronger, we are happy to do so.’

We probably don't need to worry about the wine tasting like oregano. Pelzer: “We worked with oregano throughout the season last year, and there is no oregano flavour in the wines.”