This project was planned to determine if co-infections of grapevine leafroll associated virus-3 (GLRaV-3) and grapevine virus A (GVA) lead to sudden vine collapse (SVC) on Freedom rootstock and to identify rootstocks that might be more resistant. In addition, we planned to determine if SVC was spreading within vineyards in a pattern consistent with the ecology of mealybugs, known vectors of GLRaV-3 and GVA. Eighteen blocks in 12 vineyards, located in five different counties, were identified in 2021 as having clusters of vines characteristic of SVC. 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. However, at the block level, this positive correlation was only significant in two out of 18 blocks. 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. Therefore, GLRaV-3 and GVA co- infections were not limited to SVC clusters. To determine if time was the missing factor in a positive correlation between SVC and co-infection by GLRaV-3 and GVA, we re-surveyed GLRaV-3/GVA asymptomatic vines in 2022 and 2023. None of these vines had developed SVC symptoms but only eight of the original 18 blocks were still in place in 2022 and three by 2023. Temporal and spatial analysis of SVC incidence in the remaining eight blocks indicated that SVC progressed in varying rates in five of the eight blocks from 2021-2022 before they were removed. In two of remaining three blocks, SVC incidence decreased from 2021-2023 and in the last block, yearly SVC incidence was variable. Spread occurred via a common dispersal mechanism, but at varying rates. SVC distribution within blocks was aggregated but not to the same extent as observed for GLRaV-3 epidemics. The rootstock field trial was planted in randomized blocks in the UCD Armstrong Field Station in summer 2022 and graft-inoculated in 2023 with dormant buds from vines positive for GLRaV-3, GLRaV-3 and GVA, GLRaV-1 and GVA, or GLRaV-2 and GVB, in addition to virus negative controls. A subset of graft-inoculated vines was tested by RT-qPCR in November 2023 to verify their virus infection status. The results indicated that the majority of these vines were positive for the viruses with which they were graft-inoculated.