A comprehensive review of Grapevine Pinot Gris Virus (GPGV)

Abstract

A comprehensive review of Grapevine Pinot gris virus (GPGV) was undertaken to become better informed of the risk of the virus to Australian viticulture. Gaps in the knowledge of GPGV biology and epidemiology and its impact on Australian viticulture were identified. Recommendations were developed for future research in Australia to improve our understanding of GPGV in the Australian environment and development of management strategies that are relevant locally. Preliminary management strategies were identified that can be implemented throughout the Australian viticultural supply chain to minimise risk of GPGV. A small "Preliminary Sampling Trial", which assessed sample type, amount and timing for accurate GPGV detection, identified diagnostics tests to support these management strategies.

Summary

This project was initiated in response to industry concern about the detection of Grapevine Pinot gris virus (GPGV) in several wine grape varieties (2016, 2017) and one table grape variety (2018) in three Australian states. The overarching purpose was to become informed and:

• Understand the risk of GPGV to Australian grape and wine industries
• Empower the nursery and vine improvement sector to adopt appropriate management strategies to maintain high-health status of planting material and minimise spread of GPGV
• Empower growers to manage infected blocks and to minimise spread of GPGV
• Identify gaps in knowledge of the biology and epidemiology of the virus and identify priorities for future research and development about GPGV in Australia. 

Significant research has been undertaken in Europe, especially in Italy where GPGV was first described and associated with a disease called grapevine leaf mottling and deformation (GLMD) affecting the cv. Pinot gris in 2012. The association between GLMD and GPGV is complex and remains unclear. A review of the literature identified that GPGV infects a broad range of wine, table and rootstock varieties but GLMD is reported to affect only some of these. Some varieties that can be affected by GLMD do not always show the disease, even though they are infected by GPGV. Overseas, different groups of GPGV strains are often associated with either symptomatic or asymptomatic grapevines, but the relationship between GPGV strain and symptom expression does not always hold. Reduced yield and growth is reported in some varieties that are affected by GLMD. GLMD, associated with GPGV, is not known in Australia, although no extensive surveillance has been done for the virus or the disease. Therefore, it is not known if GPGV impacts production or vineyard sustainability in Australia.

Infected Vitis vinifera and rootstock propagation material has led to the global dispersal of GPGV and the virus is now known to occur in grapevines in Asia, Europe, North America, South America and Australia. Although GPGV may have been present in a few countries prior to 2003, the movement of GPGV globally, and especially in Europe, is thought to have occurred since 2003. The introduction of GPGV into Australia is unknown, but this event is more likely to have taken place after 2003, in line with the spread into other countries and before 2014, when GPGV diagnostic testing was introduced by Australian post entry quarantine.

Research suggests that GPGV can spread rapidly once it is introduced into a new region or vineyard and it has been demonstrated that grape leaf bud and blister mites (Colomerus vitis) can transmit GPGV between grapevines. These mites are prevalent in many Australian grape growing regions and may lead to the broader dissemination of GPGV if they are not adequately controlled. GPGV has been detected in several alternative plant species in Europe that also occur in Australian vineyards, however, the importance of these alternative hosts as a reservoir from which mites can acquire and transmit GPGV to grapevines is unknown.

GPGV is detected using molecular methods such as reverse transcription-polymerase chain reaction (RT-PCR) and many tests have been developed. As a part of this project a “Preliminary Sampling Trial”, which assessed sample type, amount and timing for accurate GPGV detection, showed that reliable GPGV detection by RT-PCR is affected by genetic diversity amongst GPGV strains. The results supported the verification of four RT-PCR assays developed in Italy and these tests are recommended for detection of GPGV in Australia until further work is done to assess the genetic diversity of Australian GPGV strains to inform accurate diagnostic testing. In the “Preliminary Sampling Trial” GPGV was detected in autumn, winter and spring in one variety grown in a cool climate and another variety grown in warm climate. Further work is required to assess test reliability in summer at both sites. There is also a need to conduct a similar trial over several seasons using more varieties to confirm the most reliable time point to conduct GPGV testing.

Literature on current management strategies for GPGV and GLMD for the vineyard or nursery sectors is limited. However, preliminary management strategies that can be implemented immediately to control spread of GPGV in Australia include:

• Incorporation of GPGV testing in certification schemes to facilitate movement of high-health pathogen-tested material in which important viruses, including GPGV, have not been detected
• The use of grapevine tissue culture and/or an insect-proof screenhouse to hold high-health germplasm can minimise risk of infection
• Virus elimination can be undertaken to produce virus-free mother grapevines
• Bud and blister mite control in all vineyards, including nurseries.

A pest risk analysis was undertaken to assess the risk of entry, establishment and spread of GPGV in Australia. The assessment of consequences step was not completed because there are significant knowledge gaps about GPGV biology, epidemiology and the full impact on production and sustainability both overseas and in Australia. However, the information was used to identify critical control points and control measures within the nursery and vine improvement sector and the broader wine and grape industries for maintenance of vine health and to minimise risk of GPGV.  This review identified the following gaps in the knowledge of GPGV in Australia:

1. The geographic and varietal distribution and prevalence of GPGV and GLMD
2. The most appropriate field sampling strategy and reliable diagnostic methodologies for GPGV detection
3. The genetic diversity of GPGV and its impact on detection and symptom expression
4. Varietal response to GPGV infection on its own and with other viruses
5. The agronomic impact of GPGV and GLMD in Australia
6. The Australian vectors of GPGV and their management
7. Identification and role of alternative hosts
8. Developing GPGV free germplasm.

Future research to address these gaps is required so that adequate management strategies, that are specific for Australia, can be developed to control and minimise risk from GPGV and which can be implemented throughout the Australian viticultural supply chain. In reviewing the Australian response to the Australian GPGV detection it was also identified that there is need to foster and retain current and future knowledge, capacity and capability in pest and disease research for Australian viticulture to support the day to day management of endemic pests and disease and to be able to adequately respond to exotic pest interceptions and incursions.