Viticulture - Management and Mitigation of Impacts from Grapevine Red Blotch Virus Session

Time Since Grapevine Red Blotch Virus Infection Affects the Extent of Grapevine Red Blotch Disease on Grapes and Wine in Vitis vinifera Merlot

Raul Cauduro Girardello,* Cristina Medina-Plaza, Arpa Boghozian, Lik Xian Lim, Catherine Rout, Morgan Rosenberg, Anji Perry, Teresa Erickson, Cristian Olaya, Mysore R Sudarshana and Anita Oberholster

*University of California, Davis, One Shields Avenue, Davis, CA, 95616, rgirardello@ucdavis.edu

The impact of grapevine red blotch virus (GRBV) on grape and wine quality is of great concern in the United States. Our research group has demonstrated that grapes infected with GRBV are lower in sugar and anthocyanin concentration when compared with those from GRBV-free grapevines, resulting in wines negatively affected chemically and sensorially. In addition, it was demonstrated that both season and grape variety play an important role in GRBV impacts in grapes and wines. The virus infection is not curable and grapegrowers’ first option is generally rouging the infected vines once symptoms are visible to avoid virus spread, as well as to obtain only high-quality grapes in the vineyard. However, it is not known if the time since the grapevine is infected can influence the impact of the disease. This study evaluates the impact of GRBV during multiple years since its infection. Grape berries from Vitis vinifera L. Merlot grapevines infected by GRBV in 2016, 2019, 2020, and 2021 were harvested in 2021 in a commercial vineyard in Paso Robles, CA at maturity and analyzed for chemical (total soluble solids [TSS], pH, titratable acidity [TA]) and phenolic composition by reversed-phase high-performance liquid chromatography (RP-HPLC). Wines were made in triplicate using standard winemaking protocols at the Experimental Winery at UC Davis and analyzed for basic chemical composition as well as for phenolic and volatile compounds by RP-HPLC and headspace solid-phase microextraction gas chromatography-mass spectrometry, respectively. Descriptive analyses evaluated wine sensory attributes. Grape composition analysis at harvest (TSS, pH, and TA) demonstrated that the time since virus infection did not affect GRBV’s negative impact, except for yeast assimilable nitrogen. However, wine composition was substantially affected, depending on how long the grapevines had been infected by GRBV, especially phenolic and volatile composition. Descriptive analysis also demonstrated that panelists could distinguish wines based on the year of infection.

Funding Support: USDA-NIFA-SCRI, J. Lohr Vineyards & Wines, Brazilian Government (CAPES)

Christian Mandelli | Laurent Deluc*

Spray-Induced Gene Silencing as a Strategy for Grapevine Red Blotch Virus Control

Christian Mandelli and Laurent Deluc*

*Oregon State University, 1500 SW Jefferson Way, Corvallis, OR, 97331, laurent.deluc@oregonstate.edu

The lack of targeted approaches to control grapevine red blotch virus (GRBV) presents a significant challenge to the wine industry. Costly vine removal remains the only practical option, underscoring the urgent need for alternative strategies. RNA interference (RNAi) is a conserved defense mechanism in plants triggered by viral infection; however, the molecular interactions related to RNAi between GRBV and grapevine remain poorly understood. We recently identified nine GRBV genomic “hotspots” targeted by the plant’s RNAi machinery during an artificial infection of an infectious clone of GRBV using Agrobacterium tumefaciens. Based on these findings, we evaluated hotspot-derived dsRNA (HS-dsRNA) for their silencing efficacy in GRBV(+) tissue-cultured plants, demonstrating a significant reduction in viral gene expression at 12 and 24 days postapplication. Expanding on this, we investigated dsRNA’s systemic uptake and silencing potential targeting GRBV in greenhouse-grown plants. RNAi can be enhanced through spray-induced gene silencing (SIGS), which involves applying double-stranded RNA (dsRNA) by foliar spray. This non-transgenic and sustainable approach provides a promising strategy to control viral infection. Fluoro-labeled dsRNA applied via low-pressure spray was detected in the vascular bundles of leaf petioles within 48 hr, with systemic movement observed by 7 days posttreatment. Fluorescence persisted through 14 days, indicating dsRNA sustained transport. To assess silencing efficiency, GRBV(+) plants were treated with naked HS-dsRNA or HS-dsRNA complexed with carbon dots to enhance uptake and stability. Viral gene expression was quantified by qPCR at 1, 2 and 3 mo postapplication, with additional long-term evaluations underway to determine optimal application frequency for sustained suppression of GRBV replication. Our results provide the first experimental validation of dsRNA-induced RNAi for viral suppression in grapevine. Ongoing analyses will refine dsRNA formulation and treatment strategies, advancing the feasibility of SIGS as a sustainable alternative for GRBV control.

Funding Support: Oregon Wine Board

Joseph DeShields | Achala KC | Alexander Levin*

Potassium Application Improves Fruit Total Soluble Solids in Grapevine Red Blotch Disease-Affected Grapevines

Joseph DeShields, Achala KC and Alexander Levin*

*Southern Oregon Research and Extension Center, Oregon State University, 569 Hanley Road, Central Point, OR, Central Point, OR, OR, 97502, alexander.levin@oregonstate.edu

Grapevine red blotch disease (GRBD) is characterized by impaired photosynthetic activity, inhibited sugar translocation, and interveinal blotching on the leaves. These symptoms negatively affect fruit quality, resulting in lower sugar and anthocyanin concentrations in the fruit, making mitigation strategies essential. Given the established importance of potassium (K) in phloem loading and transport of sugars, it was hypothesized that soil and/or foliar applications of K could potentially alleviate the negative effects of GRBD on fruit ripening. In 2023 and 2024, two rates of soil-applied K, 56 kg/ha (medium) and 112 kg/ha (high), and a single rate of foliar-applied K (5.2 kg/ha), were evaluated using a split-plot design on grapevine red blotch virus-infected Cabernet franc and Merlot grapevines. A significant increase in sugar per berry (p = 0.024) was observed in response to increased soil-applied K in Merlot only. However, in both years the medium and high rates of soil-applied K increased pH in Merlot by 1.7 and 2.3% and in Cabernet franc by 4.7 and 5.8%, respectively. Both rates of soil-applied K also significantly increased GRBD symptom expression on Merlot at harvest in both years (p = 0.005), but only the high rate had a significant effect in 2024 (+7.7%; p = 0.024). In both years, foliar K had a significant effect on total soluble solids (TSS) in both cultivars. TSS averages increased by 5.8% in Cabernet franc (p = 0.010) and 3.5% in Merlot (p = 0.038). Although negative effects from soil-applied K treatments were observed on GRBD symptom severity in Merlot, this study demonstrates the effect of foliar K on increasing fruit TSS without compromising fruit yield components. However, the effects of these treatments on pH should be considered in cases where high pH levels may already be an issue.

Funding Support: OSU-Agriculture Research Foundation

8:15 am – 8:35 am	Time SinceGrapevine Red Blotch Virus Infection Affects the Extent of Grapevine Red Blotch Disease on Grapes and Wine in Vitis vinifera Merlot Raul Cauduro Girardello, University of California, Davis
8:35 am – 8:55 am	Spray-Induced Gene Silencing as a Strategy for Grapevine Red Blotch Virus Control Christian Mandelli, Oregon State University, Corvallis
8:55 am – 9:15 am	Potassium Application Improves Fruit Total Soluble Solids in GRBD-affected Grapevines Joseph DeShields, Oregon State University, Central Point

Viticulture – Management and Mitigation of Impacts from Grapevine Red Blotch Virus Session

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