Pinot Gris Virus in More Napa Vineyards

European grapevine imports also test positive for virus; erineum mite population increases

by Ted Rieger
wine vineyard pinot gris fanleaf virus grapevine arizona
This grapevine in Napa Valley displaying chlorotic leaf mottling tested positive for a mixed infection of grapevine Pinot Gris virus (GPGV) and grapevine fanleaf virus during a statewide survey for GPGV started in 2016. (Photo courtesy of Foundation Plant Services)
Davis, Calif.—Grapevine Pinot Gris virus (GPGV) has been detected in vineyards in 14 locations in Napa County, but not in other California vineyard locations to date, based on initial findings from a statewide survey started in 2016 and funded through an American Vineyard Foundation research grant. These findings and an update on GPGV were presented by Dr. Maher Al Rwahnih, diagnostic and research lab director at Foundation Plant Services (FPS) at the University of California, Davis, during a day-long extension class, “Current Issues in Vineyard Health,” held Nov. 29 in Davis. 

Disease symptoms associated with GPGV were first observed in vineyards in Italy’s Trentino Alto Adige region in 2003. In 2012, GPGV was formally recognized as a new virus by Italian researchers. Disease symptoms include chlorotic leaf mottling, leaf stunting and deformation, delayed vine growth, stunted canes and reductions in yields. In European vineyards where the virus is present and vines are symptomatic, there have been reported impacts on grape quality and plant growth. Italian researchers are studying the effects of GPGV on grape ripening and grape and wine chemistry and quality. (See "A New Disease in Italian Vineyards.")

Symptomatic and asymptomatic GPGV has been reported in wine and table grape varieties in a growing list of countries that now includes: Italy, France, Germany, Greece, Turkey, Spain, Portugal, Slovenia, the Czech Republic, Slovakia, Croatia, Serbia, Montenegro, Bosnia, Macedonia, Ukraine, Romania, Republic of Georgia, China, South Korea, Japan and Canada as well as the United States.

Grapevine viruses are commonly named based on the grape variety where they are first detected, as was the case with GPGV. The virus and its symptoms were initially associated with Italian plantings of Pinot Gris/Grigio, Gewurztraminer, Pinot Blanc, Pinot Noir, Tocai Friulano and Glera (a grape used for Prosecco), however, GPGV has been detected in a wide range of both red and white wine grape varieties in European vineyards growing Bordeaux varieties and other Italian red varieties.

GPGV in California
In 2015, commercial testing lab Agri-Analysis, based in Davis, reported that seven vines from four separate vineyards tested positive for GPGV in Napa Valley that included selections of Cabernet Sauvignon, Cabernet Franc and Chardonnay. This is believed to be the first reported detection of GPGV in commercial vineyards in the United States. Much is still unknown about this virus, and its detection in a grapevine does not necessarily mean disease symptoms are present or that it will affect vine health. 

The American Vineyard Foundation (AVF) awarded a $120,300 research grant for fiscal year 2016-17 to study “Grapevine Pinot Gris Virus in California” with Al Rwahnih as the principal investigator. Al Rwahnih provided some initial findings from 2016 survey work for several California winegrowing locations. To date, infected vines have only been found in Napa County. No GPGV has been detected so far in vine samples from Sonoma, San Luis Obispo, Monterey, Santa Cruz, San Benito, Santa Barbara, Fresno, Madera, Merced or San Joaquin counties.

The 2016 survey detected GPGV in samples of Cabernet Sauvignon, Chardonnay, Sauvignon Blanc, Malbec, Merlot, Petit Verdot, Pinot Noir, Zinfandel, Viognier, Sauvignon Musque and the rootstock 1616C in Napa County.

In one newly planted Napa vineyard, 100% of the vine samples tested were infected with GPGV. Al Rwahnih said this vineyard was planted with rootstock in 2014 and field budded in 2015. This strongly suggests that the vine material already had the virus when it was placed in the field. 

Another finding from the 2016 survey was that the virus level (titer) in infected vines was highest in the early season (May and June) and decreased during the growing season. While this may suggest incentive to collect tissue samples in spring for testing, Al Rwahnih said testing done by FPS using DNA sequencing with qualitative Real Time-Polymerase Chain Reaction (qRT-PCR) technology can detect the virus at lower levels from samples collected later in the year. Some California commercial labs can now test for GPGV.

Co-infection with multiple viruses

Al Rwahnih said, “Biological and molecular assays suggest there may be GPGV strains or isolates that can be symptomatic and others that can be asymptomatic.” The effects of GPGV infection on grapevines are not fully known. In many cases, the virus is found in vines with multiple virus infections. Other viruses found in vines with GPGV can include: grapevine rupestris stem-pitting associated virus, grapevine rupestris vein feathering virus and grapevine yellow speckle viroid 1.  Al Rwahnih noted that of 121 Napa County samples tested from symptomatic vines for other viruses, all had a co-infection with grapevine fanleaf virus and showed symptoms associated with fanleaf. Symptoms observed in Napa vines that tested positive for GPGV and fanleaf include chlorotic leaf mottling, stunted vines and, in at least one case, red leaves.

Commercial lab Agri-Analysis, in its own testing of symptomatic vines from Napa Valley, found GPGV and grapevine rupestris stem-pitting associated virus (Syrah strain) in Cabernet Sauvignon vines grafted onto Schwarzmann rootstock.

Al Rwahnih explained that the relationship between GPGV and disease symptoms appears to be complex. Since GPGV commonly occurs with other viruses, it could be a combination of viruses, the specific isolate of the virus or other factors that affect disease symptoms and their severity.

Objectives of work under the AVF research project include:
• Field surveys for GPGV in California grapegrowing regions
• Assess the relationship between GPGV infection and symptoms
• Assess symptoms enhancement by co-infection with other viruses
• Molecular characterization of California isolates of GPGV
• Improve methods for routine GPGV detection
• Monitor the natural spread of GPGV

Erineum mite vector

Studies show that the virus can be graft transmitted. Based on recent research in Italy, the grape erineum mite, Colomerus vitis, a type of eriophyid mite found in vineyards worldwide, was also shown to be a vector that can transmit GPGV to healthy grapevines in greenhouse experiments. Research is being conducted to verify whether or not this mite can transmit the virus under field conditions.

The erineum mite has historically been considered a minor grape pest in California, but its presence has been reported more frequently in vineyards in recent years, including in Napa County and in Central Coast wine grape regions. Erineum mites feed on the underside of grape leaves in spring and summer, causing the leaf to produce blister-like galls on the leaf’s upper surface. Although this is primarily a cosmetic issue on leaves in mature vines, in younger vines, high mite pressure can lead to defoliation. Maher said in some cases, the erineum mite was present in the Napa vineyard sites where GPGV was detected in 2016.

UC Cooperative Extension Napa County viticulture farm advisor Monica Cooper said increases in erineum mite populations are believed to be related to reduced use of sulfur for powdery mildew control in vineyards, in favor of alternative fungicidal products. Cooper said three strains of erineum mites have been identified, and research is under way to monitor mite populations and better understand mite biology. She said a recent mite-control experiment indicated that a post-harvest spray of wettable sulfur applied in September or October could provide effective control of erineum mites for the following growing season.

FPS material tested for GPGV
FPS now includes GPGV with its routine panel of tests for viruses and diseases under Protocol 2010, and it is in the process of testing existing Foundation plant material at its vineyards and facilities. Of 950 vines screened to date in the Russell Ranch Protocol 2010 vineyard, zero vines tested positive for GPGV. Testing is expected to be completed for the entire Russell Ranch collection in 2017. In the older FPS Classic Foundation Vineyard block, 2,322 vines have been tested with one vine testing positive for GPGV—a selection of Touriga Nacional imported from Portugal in 1981 and planted in 2001 that showed no disease symptoms or health problems. Al Rwahnih said this Touriga selection has never been distributed to growers.

FPS also has detected GPGV in several recently imported grapevine accessions from Canada, Greece, France and the Republic of Georgia now in quarantine. However, GPGV can be eliminated by meristem shoot tip culture, a standard practice used by FPS to clean plant material prior to its propagation and release.

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