Vine Virus

25 October, 2006

By Deborah Walton and Mart Verstappen

Grapevines can be host to a large number of viruses, and contrary to popular belief, not all viruses are a problem. In the case of chardonnay Mendoza, the distinguishing feature of this vine is the presence of infection with Grape Vine Leaf Roll virus type 1. Remove this virus from the vine and it will produce a berry without any of Mendoza’s desirable characteristics. In fact, the vine produces fruit that is useless to the winemaker.

As recent headlines have trumpeted, it is Grape Vine Leaf Roll virus type 3 (GLRaV-3) that is doing the damage in vineyards in certain parts of the country.

The news regarding Grape Leaf Roll virus type 3 came about largely as a result of New Zealand Winegrowers releasing a report documenting the state of New Zealand’s vineyards in relation to viruses. NZWG hopes to guarantee export potential by eliminating viruses from our vineyards. To do this, all nurseries must agree to produce grafted plants that are virus tested.

The majority of commercial nurseries in New Zealand have actually been producing virus-tested material for between five and 10 years of their own accord, without any type of regulation. This explains why in many cases, it is older vines that are compromised by the presence of the virus.

The “New Zealand Grafted Grapevine Standard” is almost in place. This standard has been agreed upon by all commercial nurseries and specifies that all propagation materials, both rootstock and scion, are virus tested and clear of contamination. This testing regime only applies to GrapeVine Leaf Roll virus type 3 and aims to nip the problem in the bud.

Why test for type 3?

This is the most damaging leaf roll virus type and as such, has the potential to have a major economic impact on the industry. It dramatically reduces fruit yield and quality from infected vines. The New Zealand wine industry is quite simply too small to allow poor wines to be released in the marketplace.

Mart recalls doing a berry tasting from vines infected with the type 3 virus several years ago. He says, “It was like carrying out virus testing by taste.

The Sauvignon Blanc berries from infected vines were watery and lacking in that zingy, defining flavour.”

If vines have the type 3 virus, destruction of diseased material and replanting is the only option.

The vector

The type 3 virus is more prevalent in some northern viticultural areas due to the presence of its vector, the long-tailed mealybug, Pseudococcus longispinus. Marlborough is reasonably safe due to little evidence of this vector’s presence

Certain types of scale are also known to spread the virus.

The symptoms

While poor fruit quality and yield are key indicators of the presence of type 3, there are also some visual symptoms of infection. The virus can attack both red and white varieties, but white varieties may show few or none of the visual symptoms typically associated with the red varieties. These can include reddening leaves with green veins or rolled back leaves.

Testing methods

Laboratories can test for nine different types of leaf roll virus, but as already noted, type 3 is the main concern.

Testing is easy, using the ELISA method. In the nursery, cuttings are taken from every plant, and the same collection methods are used in the vineyard if the grower suspects the virus is present.

There are two distinct testing timetables, based on the seasons. Green leaves are tested in spring and summer, while woody samples are tested in late autumn and through the winter. The optimum time for testing is when an infected plant reaches its maximum virus concentration. As grapevines are deciduous, the highest concentration of leaf roll virus in a given piece of tissue is around harvest time.

Grape vine leaf roll virus type 3 is one of the most common and destructive viruses found in vineyards around the world. It’s reassuring to know that research into such diseases is being undertaken and that standards are being adopted to help ensure the future viability of our wine industry.

What is ELISA?

The name ELISA is a n acronym for enzyme-linked immunosorbent assay which is a serological test to detect vine pathogens. ELISA was first used in plant pathology in the 1970s, the technique becoming routinely used to determine the presence of a wide range of vine pathogens. Test kits are available through specialist laboratories.

Serology involves the diagnostic examination of tissue and its response to pathogens or introduced substances.

Thanks to Mart Verstappen, Technical Manager, Corbans Viticulture, for technical assistance for this article.

References

Diseases and Pests, edited by P Nicholas, P Magarey and M Wachtel, Wine titles Adelaide 2003

The Oxford Companion to Wine, edited by Jancis Robinson, 1999