Seasonal dynamics and tissue distribution of two major viruses associated with grapevine Leafroll under cool climate condition

Grapevine leafroll-associated virus- 2 (GLRaV-2) and GLRaV-3 are both (+) ssRNA viruses of the family Closteroviridae and are involved in grapevine leafroll, the most destructive viral disease affecting the global grape/wine industry. Outbreaks of the disease were recently reported in Canada, causing serious concerns to the grape/wine industry. Reliable, sensitive and timely detection is key to the control of the disease. However, information on their seasonal dynamics and tissue distribution under cool climate conditions has been rather limited. We conducted a two-year comprehensive study to elucidate the temporal variation and spatial distribution of both viruses through symptom monitoring, Western blotting and RT-qPCR. Sampling was done monthly from commercial Cabernet Franc and Chardonnay vines from May to October in 2015 and 2016. Both viral RNA and capsid protein levels in leaves remained low during May and June, steadily increased from late July, and peaked in September or October. Interestingly, young berries collected in June contained high levels of viral RNA for both viruses. As expected, the viral RNA levels of GLRaV-2 detected by RT-qPCR using primers targeting the CP region were much higher than those by using primers targeting the genomic RNA. Surprisingly, virtually the same levels of viral RNA were detected for GLRaV-3 regardless of the targeted genomic regions. To ensure accurate detection of both viruses, we recommend using young berries early in the season and leaves and cambial scrapings from late July to harvest. To our knowledge, this is the first report on the seasonal dynamics and tissue distribution of two major grapevine viruses in Canada.