At least two viruses present in Nova Scotia vineyards

by Xiaotang Yang

Grapevines are susceptible to many viruses that may affect their overall health, as well as the yield and quality of the fruit. In our Plant Health Lab at Perennia, we have tested and confirmed at least two viruses that are present in Nova Scotia vineyards – Grapevine leafroll and Grapevine red blotch.

Growers should closely identify and monitor their vineyards for these potential problems. Here is some information on both types of viruses to help growers be aware.

GRAPEVINE LEAFROLL-ASSOCIATED VIRUSES

Grapevine leafroll disease is present in all grape-growing regions of the world. It can become problematic when the quality and yield of the fruit is reduced, which will in turn affect wine quality. Grapevine leafroll disease can affect all varieties, although Vitis vinifera (European wine grapes) are often more susceptible than other Vitis species and hybrids. Certain resistant rootstocks can greatly improve a plant’s tolerance to the viruses.

This disease is associated with a group of viruses called Grapevine leafroll-associated viruses. Grapevine leafroll-associated virus 3 is the most prominent and widespread of these viruses in North America. The most distinct symptoms can be observed from late summer until the end of the growing season. Infected plants develop downward-rolled leaves, hence the name “leafroll.” For red varieties, such as Pinot Noir, mature leaves will turn a red colour while the veins stay green. The white varieties such as Chardonnay will have slight yellowing of the leaf. Infected vines may also have reduced growth, resulting in smaller canopies.

Grapevine leafroll-associated viruses are not known to be mechanically transmitted by pruning equipment or harvesters. These viruses spread primarily through commercial trading of propagation material. Insects such as mealybugs and soft scales are responsible for spreading the disease from plant to plant within and between fields by feeding on vascular fluids of the grapevines.

Vineyards should be routinely inspected for Grapevine leafroll disease. However, it can be easy to confuse symptoms of viruses, potassium and magnesium deficiency, herbicide use, and environmental effects. If the disease is suspected, diagnostic testing can be done in the lab to confirm the presence of leafroll viruses.

We suggest growers mark and closely monitor the infected vines, and replace them when possible to limit further spread in the vineyard. It is also important to treat grape mealybugs and soft scales in the vineyard to keep the secondary spreading under control.

GRAPEVINE RED BLOTCH VIRUS

Grapevine red blotch virus is another widespread disease affecting North American grape production and has become an emerging issue for Canadian growers.

Similar to the Grapevine leafroll virus, Grapevine red blotch virus can infect vinifera varieties as well as hybrids, rootstocks, and table grapes. It can cause poor berry quality through uneven or delayed ripening, higher acidity, and lower sugar (lower Brix), and lower pigment content. In some cases, the virus can infect rootstock without obvious symptoms. So, using untested rootstocks can increase the risk of spreading the disease.

The symptoms of Grapevine red blotch virus infection can often be confused with Grapevine leafroll virus disease symptoms, and potassium and magnesium deficiency. In red varieties such as Cabernet Franc and Pinot Noir, red or purple blotchy spots start to develop in mid- to late summer. In white varieties such as Chardonnay, the leaves will most likely show patchy yellow areas.

The Grapevine red blotch virus can primarily be transmitted by grafting, which is largely responsible for the long-distance dissemination of this virus. Three-cornered alfalfa treehopper is considered the potential vector of Grapevine red blotch virus within the infected vineyards.

Grapevine red blotch virus doesn’t persist in the soil, so regular monitoring and roguing are effective at preventing the disease from spreading. Grapevine red blotch virus can be detected easily at any time of the year by testing leaves or even dormant cane tissues. Such testing is an opportunity to screen tissue samples before they’re propagated.

Symptoms of grapevine viruses can vary depending on many different factors. In some hybrid varieties, vines can be infected with viruses without showing any symptoms. The only way to confirm that a vine is infected with a specific virus is to have it tested in the lab.

Visit Perennia’s Plant Health Lab website for more information on sampling guidelines and to find the form on how to submit a sample. If a virus issue is suspected, consider reaching out to me or our viticulture specialist at Perennia by email at planthealthlab@perennia.ca or by phoning 902-300-3825.

(Xiaotang Yang is the molecular biologist at Perennia Food and Agriculture Inc. based in Kentville, N.S.)