The Role of Rootstocks, Single and Mixed Infections of Grapevine Leafroll Associated Virus-3, and Grapevine Virus-A in Sudden Vine Collapse. AVF Project #2025-2561.

AVF Proj. ID: 20252561 / /Cat.: ,
Primary Investigator(s): PI: Maher Al Rwahnih, Academic Administrator, Department of Plant Pathology, University of California, Davis, CA 95616; Phone: (530) 574-5463; E-mail: malrwahnih@ucdavis.edu Co-PI: Akif Eskalen, Cooperative Extension Specialist, Dept. of Plant Pathology, University of California, Davis CA 95616; Phone: (951) 640-8851; E-mail: aeskalen@ucdavis.eduby

Summary of Progress:

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.  A 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.  All vines were tested by RT-qPCR in July 2024 to verify their virus infection status.  The results indicated that most of these vines were positive for the viruses with which they were graft-inoculated.  Negative graft-inoculated vines were regrafted in summer 2024 and tested by RT-qPCR in October 2025.  Vines were scored for SVC in July–October 2025 but no symptoms were observed.

The 2021 survey indicated that while overall, there was a slightly significant positive correlation between SVC and the presence of GLRaV-3 and GVA co-infections in the 18 blocks we sampled, the association was not complete.  We had hoped to continue to score the vines in this field survey for SVC development over time to determine if this would strengthen the correlation, but the removal of SVC blocks during our survey prevented this.  However, we are continuing this work in a more controlled environment that includes specific virus infections and nine different rootstocks.  In addition, we can control the introduction of fungal pathogens that cause trunk disease once the role of GRLaV-3 and GVA co-infections alone has been determined.  The field rootstock trial was planted in August 2022 and graft-inoculated in summer 2023.  RT-qPCR testing of all vines in July 2024 indicated the majority were infected with the correct viruses and any negative vines were regrafted and tested in summer 2025.  We scored vines for SVC symptoms in July-October 2024 and 2025, but none were observed.

We will assess vines for SVC symptoms one last time in 2026.  The 2026 evaluation occurs three years post-inoculation, a time point when SVC symptoms are most likely to be expressed on Freedom rootstock.  This will conclude the portion of the proposal aiming to determine if co-infection with GLRaV-3 and GVA alone can lead to SVC, which rootstocks are susceptible and which, if any, show reduced symptom severity and therefore might be useful for SVC management.  This vineyard can then be used to determine the possible role of fungal trunk pathogens in SVC development.

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