As part of our recent Vineyard Owner Survey, we asked how concerned you were about the potential impact of Grapevine Red Blotch virus (GRBV) on your long-term vineyard sustainability.
Feedback indicated a lack of knowledge about this virus, which is unsurprising given the virus is not present in Australia.
In this article we provide a summary on what the virus is and why we test for it at the national border to try to keep it out of Australia.
What is Grapevine red blotch virus?
Grapevine red blotch virus (GRBV) is a single-stranded DNA virus in the genus Grablovirus, within the family Geminiviridae.
GRGV, limited to Vitis spp., is the causal agent of red blotch disease that was first reported in the Napa Valley, California in 2008. It was subsequently characterised from diseased vines in 2011 and is thought to have been present for decades due to the symptom similarity to leafroll viruses.
Initially, GRBV was only known to occur in the US, but has since been reported in Switzerland, Republic of Korea, Mexico, India, Canada and Argentina. In 2021, it was also detected in Italy in a table grape accession from an ampelographic collection.
The virus has been found alone and in combination with other viruses, and affects common red and white winegrape varieties, table grapes and some rootstocks. It has been detected in grapevine collections, nursery stock and established vineyards – both young and mature.
Symptoms
Red blotch disease symptoms consist of foliar and fruit symptoms.
Foliar symptoms:
- Start showing on basal leaves in mid-summer and progress along the shoot towards the upper canopy by late in the season. The severity of these symptoms and their onset varies by variety, vineyard location and growing season.
- In red varieties where symptoms are more pronounced, the disease causes leaves to become red and blotchy with marginal reddening and red veins. In white varieties when not asymptomatic, foliar symptoms include irregular chlorotic and yellowing of leaves which can turn necrotic later in the season.
- Include compromised photosynthesis in that vines cannot conduct water effectively, leaving sugars stuck in the leaves rather than accumulating in the berries.
Fruit symptoms:
- In diseased vines include consistent delayed ripening with some bunches never fully maturing and bunches with reduced sugar content (by up to 1-2˚ baume), poor colour development and increased malic acid.
- Impacts of the virus on anthocyanins, tannins, phenolics and on the resultant flavour profile and mouthfeel, has dramatically reduced winegrape value in California; noting that these effects are greater on red varieties.
- Bunch weight may be reduced; but much is still unknown about impacts on yield by variety and rootstock.
Transmission
The most common form of GRBV spread is through human-assisted spread of infected propagation material.
The Three-Cornered Alfalfa Hopper has been shown by researchers to transmit the virus in an experimental vineyard, despite not being a grape pest. However, researchers are now considering that other arthropod species are possibly vectoring GRBV. The random nature of vine infection by GRBV noted in commercial vineyards is indicative of such insect vectors, as opposed to more linear spread of infection more akin to grapevine scale or mealybug vectors.
Confusion with leafroll virus
GRBV can be most easily confused with leafroll viruses, however some important and distinct differences occur:
- Leafroll-affected vines only turn red in and around the secondary veins of the grapevine leaf, with the primary veins and surrounding area remaining green. This infection also typically causes leaf margins to roll inwards.
- Grapevine red blotch affected vines have red primary and secondary veins, as well as red interveinal zones and show no signs of leaf-rolling.
Local management advice given to growers in California
- Use clean planting material and obtain from nurseries with certification to prove the health status of their material.
- In addition, for top-working, ensure both the scion and rootstock vine material has tested negative for the virus.
- Check vines for symptoms. Mark and monitor any suspect symptoms and record observations over time. Submit tissue from symptomatic vines and asymptomatic vines for confirmatory testing, undertaking full virus screens to eliminate the presence of other viruses that may cause similar symptoms (e.g., leafroll viruses).
- Consider rogueing positive vines especially in young plantings where this has been shown to reduce spread of the virus.