A SPECIAL GROWERS’ REPORT OF AG ALERT ® CALIFORNIA Trees & Vines
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Vineyards turn to UV-C light to fight powdery mildew By Vicky Boyd A Thorvald autonomous robot applies ultraviolet-C light to grapevines at night. Winegrape growers increasingly look to the nonchemical disinfection systems to manage powdery mildew in vineyards.
Silver Oak made an initial fungicide application in April because of a delay getting started with the robots. It then continued with UV-C applications every four days for three months through veraison, or when grapes begin to ripen, in July. In the sauvignon blanc block, Peters said he also tested the UV-C against botrytis bunch rot by continuing to apply it to half the vines after veraison until three to four days before harvest. In a high-pressure disease year such as 2026, Peters said he was impressed by the cleanliness of the fruit. “It’s nice when you try something and trial it and it does what it’s supposed to do,” he said. Silver Oak’s winemaker is fermenting the fruit and will determine whether the light treatments affected wine quality, and if so, how. Silver Oak normally sprays at night, so Peters said making the transition to robots didn’t affect scheduling. One supervisor at each site was trained on the technology and oversaw each robot pair. “Over the last 10 years with all of the technology, some crew members have been wary,” he said. “In this particular situation, you come in and train somebody how to use the iPad, and it was really empowering.” Calling powdery mildew one of their biggest pest concerns, Joseph Brinkley, senior direc- tor of regenerative organic farming for Bonterra Organic Estates in Hopland, said the farm is always looking for ways to continue innovating, and the UV-C technology fit the bill. The See UV-C, Page 7
Nonchemical, ultraviolet disinfection systems originally designed for industrial and medical uses have begun to take root in winegrape vineyards to manage powdery mildew. Norway-based Saga Robotics introduced its Thorvald autonomous ultraviolet-C light-application robot to Central Coast winegrapes three years ago. This season, the company expanded to the North Coast and treated about 2,600 acres statewide, said Caine Thompson, Saga’s general manager for U.S. operations based in Paso Robles. Applied twice a week at night by the robotic platform, the 254-nanometer low-dose UV-C light disrupts the powdery mildew’s DNA without damaging plant tissue. If the application is made during daylight, the fungal organism can harness blue light from the sun to make a damage-repairing enzyme. By applying the UV-C to grapevines and fruit at night and allowing three to four hours of post-treatment darkness, the organism can’t repair the damage and dies. Norm Peters, director of vineyard operations for Oakville-based Silver Oak, said he was attracted to the system because it met the company’s sustainability priorities of putting people first and producing quality vines and wines. Before using it this spring, Peters researched the Saga system and talked to Central Coast colleagues who had used it the past two seasons. In 2026, Silver Oak tested two Thorvald robots in a 10- to 11-acre pinot noir block near Philo and two in a similar sized block of sauvignon blanc near Oakville.
6 Ag Alert October 7, 2026
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