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Tunnel Vision: Thoughts on UVC in Viticulture
- Mark Krasnow
- Apr 9
- 5 min read
Updated: Apr 10
Mark Krasnow and Karen Peterson
Rays of hope?
As viticulturist and growers, we all want to reduce our reliance on chemical inputs, and to prevent the spread of pesticide resistance in harmful species. The highest chemical inputs in viticulture, by far, are fungicides to control powdery mildew.
A promising technology, that first came to my attention in 2019 with work being conducted at Cornell University, used UVC rays to kill powdery mildew growing on leaves in the vineyard. UVC wavelengths (100nm -280nm) are filtered out by the atmosphere, so are never experienced by organisms on Earth, like the longer UVB and UVA wavelengths that can cause sunburn and skin cancer. UVC rays are even more damaging to the DNA than UVB or UVA, and because of their germicidal activity, they are commonly used in laboratory sterile hoods to disinfect surfaces. In fact, I used short periods of UVC irradiation of grape cell cultures in my PhD thesis to induce and characterise their biochemical defence responses, including the production of pigments and antifungal compounds.
The Cornell group created an irradiance tunnel with the inside covered with UVC lamps and reflectors. This tunnel was used for field evaluations of the efficacy of UVC to replace, or at the very least reduce, fungicides in Viticulture. But I’m getting a bit ahead of myself…
Before we get into the results and the implications for viticulture, I want to give just a little background on both powdery mildew and UV radiation.
UVC - Understanding the Vulnerability of Conidia (and Powdery Mildew in general)
Powdery mildew is a fungus that feeds on living tissue. It grows on the surface of the leaf and sends feeding structures into them through their stomata, located on the undersides of the leaf. The fungus, since it grows on the surface, can be controlled by external elements like salts, sulphur, and other fungicides. Because of its surface growth habit and lack of pigmentation, powdery mildew is also very susceptible to the DNA damage caused by UV radiation.
However, plants and organisms like powdery mildew that have to be in the sunlight have repair mechanisms to reduce the DNA damage caused by UV light. These repair mechanisms use visible light as an energy source, since in nature UV and visible light hit the leaves at the same time. Irradiating cells with UV in the dark eliminates the repair mechanism, leading to mutations that are generally lethal to cells. Theoretically, leaves are also at risk of damage, but they have pigments like chlorophyll and carotenoids that screen out much of the UV light. Powdery mildew, however, lacks such protective pigments. The Importance of UVC - Understanding Viticultural Constraints
As a recovering academic, I love research, especially applied research where the findings from the lab are tested under real world conditions. Often, the proof of concept results from the lab can't be replicated in the field. Other times, results can be duplicated, but doing so is economically or logistically non-viable at industry scale. As of now, I feel UVC falls into the second category.
UVC - Under Vampire Conditions
Given this background, you can probably see the real weakness to UVC treatments in the vineyard - they require darkness. It actually takes about four hours of uninterrupted darkness after the irradiation for UVC to kill powdery mildew, which limits the use of UVC in vineyards even further, since it can’t even be used for the entire night. To make matters even worse, the highest pressure time for powdery mildew in most grape growing regions, is when the nights are the shortest.
At our latitude in Marlborough, the shortest nights we have are about 8 hours, meaning only four hours of run time every night. At lower latitudes, such as Central Otago, the effective nightly run time would be even shorter. Given the huge number of vines and the short nights when control is needed the most, the perception that UVC could eliminate fungicides is unrealistic in our current machinery environment.
Also, the thought that a UV treatment can even substitute for a sulphur spray is incorrect. Sulphur, and other fungicides, have residual activity after spraying since they remain on the leaves if they’re not washed off. UV treatment leaves no protective residue behind. The day after a UV treatment, powdery mildew could infect a treated leaf, whereas it could not with leaves sprayed with sulphur. UV does induce antifungal compound production, but research shows leaves are still infectable after irradiation. Therefore, if UV treatment is going to substitute for anything in the vineyard spray programme, it will be eradicates used after infections pop up, like bicarbonates, peroxide, or ozone. Ideally, these products aren’t needed in a well managed vineyard.
A research programme, called "A Lighter Touch" looking into chemical reduction across many agricultural products, started in New Zealand this past season. As part of this programme, they created a UVC irradiance platform to run down vineyard rows. This rig only illuminates from the sides, unlike the Cornell tunnel. The study will assess the efficacy of reducing or eliminating fungicide sprays by utilising UVC. As of the publication of this blog the results of the trial have not been released, but I will revisit this topic once they are disseminated.
UVC - Utilise Vigilant Critiques
It is easy to see this tech through rose coloured (and hopefully UV resistant) glasses - how could reducing chemistry be bad?
Results from the field show efficacy (see the resources at the end of this blog), but getting good control required twice a week treatment, four times the passes required now
Light pollution from the rigs, especially open panels like the ones used by the "A Lighter Touch" Programme. Tunnels reduce, but do not eliminate light pollution
For Organic and Biodynamic growers who may use biological control methods - is there a negative impact on colony development which is imperative to their efficacy?
Effects on beneficials. We used UVC lamps to sterilise our fume hoods. UVC could potentially be more harmful for beneficial species than fungicides. As a new technology, this information is still unknown.
Health and safety. Eye protection would be critical, but given the power of UVC, full protective gear might be necessary, especially with an open rig.
UVC - Until Value Confirmed...
I do envision a future where drones or other equipment are able to irradiate vines frequently enough so that fungicides aren’t needed, or are needed barely at all. However, we are a long way from that future. While UVC shows some real promise, it is definitely not a game changer for Viticulture any time soon. In my opinion, other avenues, such as biopesticides, biostimulants and bioelicitors, are much more likely to dramatically reduce our synthetic fungicide use in the short term than UVC treatment will.
The bright side
However, UV treatment does have some potential uses, even in the short term, for vineyards.
· As an eradicant. Especially later in the season, when other eradicants might leave a residue that can taint wine
· For use around veraison for wineries that don’t want sulphur residue on fruit for fear of reductive ferments.
· In red wines, treatment of fruit could improve phenolic composition and/or pigmentation. This avenue deserves some research in my opinion.
Additional Resources
Crop Consultant article by D. Gadoury https://thorvald.leahjade.co.uk/wp-content/uploads/ProgCropConsultUVMay21.pdf
Effects of UVC on powdery mildew, downy mildew, and sour rot https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-04-22-0984-RE
Effects of UVC on powdery mildew in Chardonnay https://www.ajevonline.org/content/ajev/75/1/0750014.full.pdf
A lighter touch programme https://a-lighter-touch.co.nz/
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