W. McFadden-Smith (OMAFRA), S. Poojari and J.P. Parent (AAFC)
Symptoms of Grapevine leafroll disease (GLD), caused by a number of GLRaVs, are more apparent on red-fruited vinifera cultivars than whites and they vary based on variety of factors including environmental conditions, cultivar, age of the vines, stage and severity of (one or more than one virus) infection and viral load.
Typical symptoms of GLD in red vinifera cultivars include reddish-purple interveinal discoloration with larger veins remaining green. In later stages, the margins of symptomatic leaves roll backwards. In white vinifera cultivars such dramatic symptoms are not seen but some cultivars show yellowing and rolling of leaf margins typically at the end of the growing season. Hybrid, table and juice varieties as well as rootstocks and wild grapevines can be infected with GLRaVs but remain symptomless.
Symptoms associated with grapevine leafroll disease on red-fruited cultivars cv. Merlot (on left) and white-fruited cultivar cv. Chardonnay (on right). (Source: AAFC, SuRDC).
Although, the symptoms of GLD and Grapevine red blotch disease (GRBD) are similar, the reddish discoloration on the leaves of GRBD affected red-fruited vinifera cultivars are blotchy and in some cases the primary and lateral veins show purple to reddish discolorations. Symptoms of red blotch on white cultivars are less distinctive. No downward rolling of leaf margins was observed in GRBV infected red and white fruited cultivars.
Symptoms of red blotch on red-fruited wine grape cultivar (cv. Merlot) (Source: http://dx.doi.org/10.1080/07060661.2017.1312532)
Symptoms of GLD and GRBD started becoming apparent, especially in red varieties, in early to mid-August. GLRaVs and GRBV block the vascular tissue that’s required to move sugar from leaves to fruit which results in delayed ripening and decreased fruit quality. It is believed that virus infected vines are prone to more significant fruit quality impacts in cooler growing seasons due to early expression of symptoms than warmer seasons in which vines tends to accumulate sufficient sugar levels much before the symptoms start to fully express.
Healthy Pinot noir vine (left) beside a vine infected with GLRaV (right). Note the delayed maturity of fruit on the infected vine.
The three cornered alfalfa tree hopper has been reported vector of grapevine red blotch virus based on laboratory experiments in California. However, the epidemiological role of this and other species of phloem-feeding insect species is still under investigation. We are currently monitoring and investigating the role of these insects in possible transmission of GRBV by these insects in Ontario vineyards.
Three-cornered alfalfa hopper (Spissistilus festinus Say.), a potential vector of grapevine red-blotch virus
Once a vine is infected with virus, it cannot be cured. There are conflicting reports in the literature regarding the effectiveness of reducing crop load, applying additional fertilizer or generally reducing stress on the vine in mitigating the effects of infection. For the most part, there doesn’t seem to be a lot that can be done to help increase Brix in infected vines.
Because it isn’t possible to identify infected vines after the leaves drop, go through the vineyard and spray paint the trunks of vines with virus symptoms when symptoms become apparent. Fluorescent orange paint seems to hold up best through the winter. This will allow a good visual of what proportion of the vines are infected before you make the decision whether or not to remove vines.
The only way to confirm that a vine is infected with leafroll, red blotch or both viruses is to have it tested by an accredited lab. The links below provide information on sample collection and submission from labs providing testing:
Pest Diagnostic clinic, University of Guelph: http://afl.uoguelph.ca/submitting-samples
A and L labs: http://www.albiologicals.com/?page_id=222
Click for a pdf of this article: Grapevine viruses Sept 2017