The Role of Rootstocks and Single and Mixed Infections of Grapevine Leafroll Associated Virus-3 and Grapevine Virus A in Sudden Vine Collapse

Eighteen blocks in 12 vineyards, located in five different counties, were identified as having clusters of vines characteristic of sudden vine collapse (SVC). The vines included eight scion varieties, two rootstock varieties, and one own-rooted. Leaf samples were collected from both symptomatic and asymptomatic vines within SVC clusters. In addition, asymptomatic vines outside of the SVC cluster were included as an indication of the overall infection status of the block. Samples were processed and tested by RT-qPCR for GLRaV-1, -2, -3, GVA and GVB. Eighty-seven percent of all vines were positive for GLRaV-3, 73% were positive for both GLRaV-3 and GVA, and 26% were positive for both GLRaV-3 and GVB. All GVA and GVB infections were co-infected with GLRaV-3. One vine was positive for GLRaV-1 but no GLRaV[1]2 positive vines were detected. A comparison of GLRaV-3 and GVA infection rates in symptomatic versus asymptomatic vines within SVC clusters indicated that overall, the rates were slightly but significantly higher in symptomatic vines. Analysis at the block level indicated that this positive correlation was only significant in two out of 16 blocks due to the constraints of small sample size. Therefore, GLRaV-3/GVA co-infection rates in symptomatic versus asymptomatic vines were not significantly different in most of the blocks we sampled. A comparison of GLRaV-3/GVA infection rates in asymptomatic vines within and outside SVC clusters indicated that only two blocks had significantly higher co-infection rates in asymptomatic vines within clusters. Conversely, one block had significantly higher infection rates in asymptomatic vines outside the SVC cluster. These observations, coupled with the overall high percentage of vines positive for GLRaV-3 and GVA indicates that the blocks we selected for this study had high GLRaV-3/GVA co-infection rates despite the lack of disease symptoms outside SVC clusters. Fewer GVB positive vines were detected, and these infections were almost evenly divided between symptomatic and asymptomatic vines within SVC clusters, and between asymptomatic vines within and outside the SVC cluster. To track the progression of SVC in these 18 blocks over the next two years, a group of 300 SVC symptomatic and asymptomatic vines within SVC clusters were mapped by row and vine location. For the rootstock field trial, approximately 80 vines each of nine different rootstocks were propagated from dormant cuttings and chip-bud grafted with one bud from Pinot gris 09. These vines were transplanted into 1-gallon pots and are being kept in a FPS screenhouse until spring 2022. In addition, vines positive for GLRaV-3, GLRaV-3 and GVA, GLRaV-1 and GVA, and GLRaV-2 and GVB have been identified and analyzed by high throughput sequencing to verify their infection status. A field site has been identified at the UCD Armstrong Field Station and prepared for planting vines in the spring 2022.