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A team of researchers at Oregon State University’s Oregon Wine Research Institute at Oregon State University have been conducting research and outreach to address the effects of Grapevine Red Blotch Disease on vineyards in Oregon since 2016. The disease was recently identified by scientists and found throughout vineyards on the West Coast, including Oregon. The disease has potential to impact berry ripening and decrease wine quality, creating a significant concern for Oregon’s premium wine producers. Research to date provides industry with information about disease presence in the state and impacts on vines, but continued research is needed to develop management options to cope longer-term. 

A single leaf of this Red Blotch Disease infected Pinot noir vine shows symptoms of the disease in September,
before harvest. The vineyard is located in Oregon's Willamette Valley. Photo courtesy of Patty Skinkis.

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Grapevine red blotch disease (GRBD) is a concern for many growers and winemakers in Oregon’s wine industry. Although first expressed in Cabernet Sauvignon in Napa Valley, California roughly a decade ago, it has since been characterized as affecting many grape varieties along the West Coast. While producers fear that infected vineyards will require removal and replacement of their vines, winemakers worry about the virus is affecting wine quality. Scientists at Oregon State University are researching the effects that GRBD will have on the Oregon wine industry and are exploring management techniques for growers and winemakers to help subdue GRBD effects.

Grapevine Red Blotch Disease is caused by a virus and is transmitted through infected plant material (bud wood or plants) at grafting or propagating from infected plants. By purchasing certified plants, also known as “clean plant material,” that are free of all known viruses, growers can minimize risk of planting vines already infected with the virus. However, researchers believe that there may be insect vectors that can transport the virus from infected vines to healthy vines. Some lab studies have demonstrated transmission from insects of the treehopper and leafhopper families, including the Three-Cornered Alfalfa Hopper and the Virginia Creeper Leafhopper. However, transmission has not been proven in commercial vineyards. Oregon State University scientists have focused on two main areas for studying GRBD vectors: 1) investigating species of tree and leafhoppers that exist in Oregon, and 2) looking for other potential insect vector species that exist within infected vineyards thought to have virus spread.

USDA-ARS plant pathologist, Dr. Bob Martin has been conducting GRBD research for more than five years. He has been investigating different potential vectors and identifying what time of year transmission occurs. For the past two seasons, he has collaborated with USDA-ARS entomologist, Dr. Jana Lee to collect and analyze thousands of potential insect vectors and test them in the lab by way of an exposure test to see if they will transmit the virus. They have conducted these tests with insects found in vineyards throughout western Oregon by placing them with vines infected with GRBD for one week, then introducing them to clean plant material for another week. By testing the clean plant material that has come in contact with the insect, they can determine if virus transmission occurs, but it takes time. The virus is known to have a long latency period from infection to receiving a positive diagnosis by lab testing (PCR assay).

Oregon State University’s Dr. Vaughn Walton, a professor and entomologist in the Department of Horticulture, is also working on identifying insects that can vector the virus. Walton’s research focuses on determining environmentally sustainable pest management strategies for agriculturalist by researching insect physiology and biological behaviors while also learning new information about how insects interact with the environment. Dr. Walton has worked extensively with mealybugs, a known vector of grapevine leafroll virus, another important virus of vineyards; he used this expertise to guide his GRBD vector experiments. His lab has been working with insect vibration signals to help look for insect vectors of GRBD. Insects communicate with vibration signals. These signals are recorded and then played back within the field to attract other insects. Rivalry signals may also be used as a repelling agent. This technique has been used as a trapping method to determine what insects are present in a vineyard. To date the Walton Lab has not identified an insect vector that can continually repeat any positive transmissions of the virus.

Dr. Alexander Levin, assistant professor and viticulturist at OSU’s Southern Oregon Research and Extension Center (SOREC), has been working with GRBD since 2016 alongside, Dr. Achala KC, assistant professor and plant pathologist, also located at SOREC. Dr. Levin’s research focuses on characterizing vine responses to the virus infection through interaction of environmental stresses, primarily water deficit. With southern Oregon’s warmer climate, vines affected by GRBD have been observed showing greater signs of stress. He has been evaluating different vineyard management strategies such as increased fertilizer and water inputs, reducing crop load, and the use of different rootstocks to determine impact on GRBD infected vine responses. Drs. Levin and KC found water deficit reduces fruit quality in GRBD infected vines. Keeping vines well-watered, growers may be able to mitigate the negative fruit quality effects of the virus. For the crop load experiments, Dr. Levin found that a reduced crop load showed only a slight decrease in negative effects compared to vines with an average to heavy load. He continues his work by adjusting his experiments to create less stress on the plant, including more fertilizer, more water, and less crop. In general, they have found that less stressed vines expressed less disease severity, which means more healthy leaves in a vine canopy. They have two seasons of data so far and are finding greater differences in fruit ripening between well-watered and less irrigated treatments. However, they continue to conduct additional seasons of research to understand long-term impacts.

Dr. Achala KC has been researching ways to improve GRBD disease diagnosis. To confirm that vines are infected, growers submit plant samples to a plant disease testing lab to determine if the virus is present. Dr. KC is determining the best types of grapevine tissue to collect to increase the accuracy of disease diagnosis. She has found that the older petioles on leaves at the base of the vine’s canopy provide a more accurate test result on GRBD diagnosis. The samples can be collected as early as fruit set in a mature vineyard. However, late season sampling is best, and she recommends sampling at or after harvest is when visual symptoms are present. Finding the right plant tissue to sample will help researchers to set up research trials earlier in the summer, rather than waiting until late in a crop year to see symptom expression to identify GRBD positive vines for research. In many cases, vines may be infected but do not show symptoms, resulting in inconsistent data for researchers trying to understand the virus. The research team continues to explore virus detection methods to be able to correlate virus quantity and symptom expression or severity of symptoms.

Dr. Patty Skinkis, Professor and Viticulture Extension Specialist, is working to characterize GRBD symptomology of vineyards in the Willamette Valley. As an Extension specialist, she always has the producers in mind and aims to find practical and sustainable solutions. Since there is no way to cure a grapevine virus, the only option to eradicate virus is to rip out the vineyard. However, this is not always possible or practical given the amount of financial investment in a commercial vineyard. Therefore, Dr. Skinkis is looking for ways to manage the virus and mitigate its effects related to delayed ripening and lower fruit quality. Her lab has studied GRBD infected vineyards since 2017. By tracking incidence severity of the symptoms, her lab hoped to find important physiological performance impacts to target with vineyard management treatments. There have been minimum impacts of the virus on the vineyard sites researched in the Willamette Valley. However, she began to investigate different management techniques to help minimize virus effects, focusing on enhancing fruit ripening. The trials included a leaf removal experiment that evaluated removing more cluster zone leaves earlier in the season to help increase fruit color and phenolics compared to industry standard practice. Her lab also conducted an abscisic acid (ABA) trial that involved applying ABA to the fruit at the beginning of ripening to enhance the ripening process. This work was in cooperation with OSU food science researchers, including Dr. James Osborne, Extension Enologist, Dr. Elizabeth Tomasino, sensory scientist, and Dr. Michael Qian, flavor chemist. Results to date show that early season leaf removal can enhance color, phenolics, and aromas. However, the ABA application was not found to be effective. The most important finding of the research is that vineyards with less abiotic stress had fewer symptoms of the virus, thus she is encouraging growers to “baby their vines” by reducing competition with cover crops or providing irrigation to mitigate the impacts of the virus.

The industry is intrigued to know how GRBD affects wine quality. For this reason, scientists from the OSU Department of Food Science and Technology, including Drs. James Osborne, Michael Qian, and Elizabeth Tomasino, have partnered with viticulture researchers to produce wines from field trials and carry out compositional analyses and sensory panels. Dr. Osborne, Enology Extension Specialist, focuses on the overall quality of the wine derived from GRBD infected vineyards as well as the fermentation dynamics that occur. Dr. Qian has been analyzing volatile aroma compounds of wines produced and has also conducted compositional analyses of phenolics. Dr. Tomasino is conducting sensory analysis of the research wines. To date, results show that there are few and inconsistent differences in wine composition, and sensory analysis by wine experts reveal little to no distinction between healthy and GRBD infected wines.

With GRBD affecting many vineyards along the West Coast, researchers are working to help identify the virus impacts and find management techniques that vineyards and wineries can provide some solutions. There are still many years of research ahead, and with the help of a $3 million USDA-NIFA grant, researchers from UC Davis, UC Berkley and OSU can continue to examine GRBD. With a $162 billion U.S. grape industry at risk, researchers are determined to set the industries mind at ease by continuing to study the virus, its effects on grapevines and wines, and identify any possible insect vectors to configure a long-term disease management plan. With the effects on wine quality research still in process, we hope to better understand impacts of the virus in the near future.
 

This article was written by Makenzie Blaylock, an undergraduate student in the Department of Horticulture and student research assistant in the lab of Dr. Patty Skinkis, Professor and Viticulture Extension Specialist.