Basavaraj Bagewadi, Adilakshmi Movva, Nagateja Natra, and Rayapati Naidu* *Washington State University, Prosser, Department of Plant Pathology, Irrigated Agriculture Research and Extension Center (IAREC), 24106 N Bunn Rd., Prosser, WA 99350 (naidu.rayapati@wsu.edu)

Virus-tested plant material is recognized as the best antidote for establishing healthy vineyards. With increasing demand for “clean” plants, there is a critical need to maintain virus-tested plant material in certified grapevine nurseries to ensure that viruses are not disseminated via plant material from nurseries to vineyards. With this objective, we have collaborated with grapevine nurseries and the Washington State Department of Agriculture to implement high sanitary standards in Certified Mother Blocks. We have adopted a composite sampling strategy for high-throughput virus indexing of leaf samples collected during the season and cane samples collected during the dormant season. Extracts from composite samples were tested initially using molecular diagnostic assays for the presence of Grapevine leafroll-associated virus 3 (GLRaV-3) and Grapevine red blotch-associated virus (GRBaV). In 2016, 2054 composite samples collected from different grapevine cultivars maintained by three nurseries were tested for GLRaV-3 and GRBaV. Names of nurseries were withheld due to confidentiality. Based on virus indexing results, only 47 of the 2054 composite samples (~2.29%) tested positive for GLRaV-3. All 2054 composite samples tested negative for GRBaV. A subset of these composite samples was subjected to high-throughput sequencing to confirm the above results and document the presence of other viruses. Testing composite samples, followed by retesting samples from individual vines if a composite sample comes up positive, was advantageous for virus-indexing large numbers of samples. Results obtained during the past three seasons showed the absence of GRBaV in grapevines maintained in Certified Mother Blocks. However, the spread of GL-RaV-3 into Mother Blocks is a concern for nurseries. Therefore, virus indexing of grapevines in Certified Mother Blocks at regular intervals is necessary to maintain high sanitary standards in nurseries and to make “clean” plant material available to growers.

Funding Support: WSU Agricultural Research Center, WSDA-Grower Assessment Fund, and WSDA-Specialty Crop Block Grant Program (K1765)