Grapevine Pinot Gris Virus found in Napa Valley
Effects on wine grape yield and quality unknown for newly identified virus
The grapevine Pinot Gris virus (GPGV) recently was identified in a number of Napa Valley vineyards after being discovered in vineyards from Korea to Canada, and widely in the Mediterranean region.
The virus was the topic of one of the sessions at the second Sustainable Vineyard Practices seminar on Pests & Diseases organized by the Napa Valley Grapegrowers, University of California Cooperative Extension and the Napa Valley Vineyard Technical Group (Vit Tech).
The seminar was held in the auditorium of Copia in Napa, the first return of the sessions to the long-closed facility since its acquisition by the Culinary Institute of America.
Napa County viticulture advisor Dr. Monica Cooper gave the presentation for Dr. Maher Al Rwahnih, associate plant scientist for Foundation Plant Services at the University of California, Davis, who couldn’t attend. The following summary is based on the presentation and Al Rwahnih’s notes.
Only identified recently
The symptoms of the grapevine Pinot Gris virus—chlorotic mottling, leaf deformation and stunting—were first noticed in vineyards in Trentino (northeastern Italy) in 2003. It was identified in 2012 by high-throughput sequencing as a new member of the Trichovirus genus. (See “A New Disease in Italian Vineyards.”)
The new virus was found not only in Pinot Gris (called Pinot Grigio in Italy) but also on Traminer (known in the U.S. as Gewurztraminer) and Pinot Noir and subsequently on other varieties including Chardonnay.
The GPGV sequence analysis demonstrated that it was closely related to Grapevine berry inner necrosis virus (GINV) but affected only the leaves, not the berries.
GINV is believed to be spread by the eriophyid mite Colomerus vitis, Pagenstecher.
GPGV has been reported in symptomatic and asymptomatic vines for wine and table grape varieties in many countries, with symptoms appearing on 16 of 17 vines tested, though also found in six of 23 vines that didn’t exhibit symptoms.
The virus has since been found in Trentino, Friuli Venezia Giulia, Veneto, Emilia Romagna and Puglia in Italy, and in Slovenia, the Czech Republic and Slovakia, Greece, France, China, Turkey, Republic of Georgia, South Korea, Canada and the United States.
The effects of the GPGV infection on grapevines are still poorly characterized. The relationship between GPGV infection and disease symptoms appears to be complex and is not yet understood.
Studies show that different factors may affect symptom expression. Biological and molecular assays using Pinot Gris and Traminer/ Gewurztraminer grapevines suggest that there may be different symptomatic and asymptomatic GPGV isolates.
A survey of different cultivars from the Trentino region in Italy showed widespread distribution of GPGV, and about 80% of the vines that tested positive for GPPV showed symptoms, but the other 20% didn’t.
In most cases, GPGV has been reported in plants supporting multiple viral infections. Other viruses found in vines with GPGV include: Grapevine rupestris stem pitting-associated virus and Grapevine rupestris vein feathering virus
Diagnosis of GPGV based on specific symptoms can be challenging, because in some cases the virus appears asymptomatic, and in other cases, it occurs as a mixed infection with other symptomatic viruses. So far, sequence-based assays are the only method used for the virus diagnosis and can be plagued with false readings.
Preliminary results obtained from transmission trials suggest that the Colomerus vitis mite can acquire GPGV and transfer it to healthy grapevines, making the mite a potential candidate vector for natural GPGV transmission. This vector infests grapevines wherever they are grown, including California.
Good news and bad news
Early research showed that GPGV could be eliminated easily by in vitro meristem tip culture and/or thermotherapy. Thermotherapy-treated vines were maintained for two to four months at 38° C or for 40 days at 34° C before meristem excision.
An obvious question to California growers is whether GPGV is present in the Foundation Plant Services Vineyards in Davis and at Russell Ranch. FPS used real-time quantitative qRT-PCR assay to test 2,014 vines altogether including 23 Pinot Gris vines.
At Russell Ranch, FPS tested 254 vines and found none infected with GPGV. At the Classic Foundation Vineyard, it tested 1,760 vines and found one infected vine that didn’t exhibit symptoms, a Touriga Nacional plant imported from Portugal in 1981. It received tissue culture virus therapy, passed quarantine tests and was planted in the FPS collection in 2001. This is believed to be the first reported detection of GPGV in the United States.
FPS also tested the 20 vines surrounding the infected vine. All tested negative, so there is no indication of active spread of the virus.
FPS now has started testing all imported and local selections in the pipeline for this virus. Testing resulted in the finding of GPGV in several imported vines from Canada, Greece and the Republic of Georgia.
GPGV also has been detected in a grapevine rootstock breeding selection originally propagated from seedlings.
Also very recently, a private virus-testing laboratory reported GPGV from three separate vineyards in Napa Valley.
Seven vines from four commercial vineyards tested positive for GPGV, including selections of Cabernet Sauvignon, Cabernet Franc and Chardonnay.
This is believed to be the first reported detection of GPGV in the Napa Valley vineyards.
FPS also has been testing stored total nucleic acid samples that had been collected from a selection of vineyards from Napa Valley and elsewhere.
In summary, GPGV has been reported in symptomatic and asymptomatic wine and table grapevines in many countries. Biological and molecular assays suggested that there may be symptomatic and asymptomatic GPGV isolates.
Recent transmission studies indicated that the eriophyid mite could be the vector of GPGV.
The existing PCR-based diagnostic methods are not reliable.
However, the effects of the GPGV infection on grapevines are still unknown and, in most cases, the virus was reported in the plants with multiple viral infections.
Some progress has been made to understand the biology and ecology of GPGV since it was first reported in Europe, but it is still not fully understood. The incidence and distribution of GPGV in California vineyards is still unmeasured, and field surveys in the main grapegrowing regions will be needed to characterize that distribution and identify possible associated symptoms. This research would document the occurrence of GPGV, further characterize the impact of GPGV in California and document the role of mixed viral infection in symptoms' expression.