Vicki Klaassen, Maher Al Rwahnih,* Neil McRoberts, Brianna McGuire, Kent Daane, and Alexis Billings
*Foundation Plant Services, Department of Plant Pathology, University of California, Davis, 455 Hopkins Rd., Davis, CA, 95616 (malrwahnih@ucdavis.edu)

Understanding grapevine red blotch virus (GRBV) spread rates and patterns in infect­ed vineyards can provide important disease management information. However, this information has been documented and analyzed in only two California vineyards. The availability of limited information on GRBV spread is largely due to the expense of testing many individual vines in infected vineyards. Therefore, when GRBV was iden­tified in 2017 in the Russell Ranch Foundation Vineyard (RRV) and reached a positive incidence of 18% by 2019, we decided to continue to document GRBV incidence and do a detailed analysis of spread patterns over time. RRV offered a unique opportunity for this study because GRBV was not introduced in infected planting material and yearly test data for more than 4000 vines was available. The specific objectives of this study were to document GRBV incidence in 2020 and 2021, use spatiotemporal analysis to analyze spread patterns from 2017 to 2021, plant and test 400 sentinel vines to characterize GRBV detection in newly-infected vines, and to survey insect populations. We conclude that GRBV was introduced into RRV from outside sources and then spread rapidly within the vineyard by an unknown vector that has a longer mean dispersal distance than mealybugs, the vector of grapevine leafroll associated virus-3. While all candidate insect vectors were present at RRV, none were associated with GRBV-infected vines. Test data from sentinel vines indicated that new GRBV infections could be detected by qPCR within a year, but low titer and/or uneven within-vine distribution would result in some unknown fraction of false negative test results. Combined, these factors indicate that in some vineyards, GRBV spread cannot be controlled by removing infected vines and insect management programs. For a foundation vineyard such as RRV, a greenhouse collection is the most feasible alternative.

Funding Support: Pierce’s Disease/Glassy-Winged Sharpshooter Board