March 15, 2023
From unpredictable weather to pests and diseases, vineyard managers contend with a lot of challenges. Sessions at the recent WineVit conference showed that Washington State University scientists are determined to help Washington’s grape growers be successful and prosperous.
In a talk covering leafroll disease management, Naidu Rayapati, director of WSU’s Irrigated Agriculture Research and Extension Center (IAREC), outlined the nature of leafroll disease and how it spreads into and within young vineyards.
Leafroll is the most complex viral disease known to infect plants, and grapevines can express symptoms differently, making it difficult to track: Red-grape cultivars display dramatic symptoms when infected, whereas white-grape cultivars often show subtle symptoms.
To add to the complexity, infected vines may not express disease symptoms until after veraison — the stage when grapes start to soften and change color, indicating that they’re beginning to ripen. Overlapping or similar symptoms can also be induced by red blotch disease and abiotic factors such as vine injury. It’s therefore crucial to test for the virus rather than relying on visible symptoms during vineyard scouting for disease management, Rayapati said.
Vineyard to vineyard
WSU Extension entomologist Doug Walsh explained that the virus spreads from vineyard to vineyard mainly through windblown grape mealybug crawlers. In controlled laboratory studies, feeding from a single infected mealybug crawler can transmit the leafroll virus up to 20% of the time.
The grape mealybug is currently managed mainly through insecticide application, but another solution may be on the horizon. Walsh highlighted the promising work of Stephen Onayemi, a PhD student in WSU’s Department of Entomology, whose pilot studies with pheromone dispensers show great potential for grape mealybug mating disruption.
Such research is critical for growers because of leafroll’s long-term implications, economic and otherwise.
“We’re not able to provide a silver bullet right now, but we are inching toward sustainable solutions for managing leafroll disease in vineyards,” Rayapati said. “Compared to 15 years ago, we have a better understanding of the disease’s complexity, the nature of the virus and the vector, and how leafroll spreads from infected blocks to healthy plantings.”
Pathogens like viruses can be avoided in the first place by sourcing “clean” plants, explained Scott Harper, director of the IAREC-based Clean Plant Center Northwest (CPCNW), during a session on controlling invasive species and managing pathogens.
Harper, an assistant professor in WSU’s Department of Plant Pathology, spoke about clean plant certification and the role of CPCNW, which tests for about 70 grapevine-infecting viruses and provides clean starting material, dormant cuttings, and propagative plants to nurseries.
He emphasized that clean plant testing is an ongoing process because there is potential for reinfection at any time.
“A plant is only as clean as the last time it was tested,” Harper said.
He concluded the session by stressing the importance of clean plant certification and rigorous testing methods.
“These systems exist for a reason,” he said. “They’re here to protect you from the next outbreak.”
Wine and vines
The conference also included a session dedicated to the science behind creating palatable wine and growing healthy grapevines despite unpredictable environmental conditions.
Devin Rippner, an IAREC-based WSU adjunct assistant professor and research soil scientist with the USDA Agricultural Research Service, explained nitrogen’s importance in the vineyard and how a happy medium of water and nitrogen application can increase vineyard profitability, prevent excessive vigor, and decrease fungal disease pressure.
During the same session, WSU Viticulture and Enology Professor Thomas Henick-Kling shared how nutrient management practices for yeast and bacteria can help avoid unpleasant odors and stuck and sluggish fermentations in wine production, emphasizing the importance of nutrient addition timing and yeast and bacteria nutrition.
Though growers may be tempted to manage vineyards on a calendar basis, an in-depth session about weather’s impact on farming illustrated why this isn’t always wise.
“Conditions can change every year,” said WSU Viticulture Extension Specialist Michelle Moyer. “When you rely on a calendar-based program, opportunities for pest and disease intervention can be missed. It’s best to focus on a phenology- and pest pressure-based calendar instead.”
One key takeaway was weather’s role in the vine growing process. Moyer encouraged the audience to remember that weather patterns can affect vine development and susceptibility to pests and diseases like grape powdery mildew, Botrytis cinerea, sour rot, and grape mealybug.
“Weather matters, especially during those critical windows,” Moyer said. “It can drive the severity of disease in your vineyard.”
Markus Keller, Chateau Ste. Michelle Distinguished Professor in Viticulture, added that grapevine phenology can be stretched out or compressed, depending on temperature and how quickly growing days accumulate.
Temperature can impact bud break and affect the seasonal vigor of the shoots, creating challenges for growers.
Next year's crop
“Farming by phenology can be complicated,” Keller said. “Whatever you do at any one stage of the growth cycle also influences what happens next year.”
A calendar-based approach to irrigation and nitrogen fertilizer application is discouraged, too.
“It’s important to stay flexible,” Keller said. “When the year throws you something different in terms of vine development, don’t manage it strictly by the calendar. Use the phenological state of the plant instead.”
The session concluded with Lav Khot, interim director of WSU’s AgWeatherNet (AWN) system, who highlighted how growers can use AWN’s network of 346 stations to gain valuable climatological and historical trend data, hyper-accurate forecasts, and other weather insights.
[Source: Washington State University]
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