Epidemiology of Grapevine Red Blotch Disease Progression in Southern Oregon Vineyards

Grapevine red blotch disease (GRBD) is an emerging disease caused by grapevine red blotch virus (GRBV). It is widespread in most United States winegrape production regions and differential spread dynamics have been reported between the regions. This study surveyed eight vineyard sites in southern Oregon over four years for the progression of GRBD incidence. The vineyards included five sites that were five years or older and three sites that were three-years-old. The disease incidence in the older blocks ranged from 2.81 to 58.78%, while in the younger blocks it ranged from 0.29 to 1.11%. Some vineyards implemented frequent disease scouting, removing infected vines and replanting. The disease incidence in these blocks remained <5% over the survey period. However, in vineyards with no roguing and replanting, the disease incidence increased nearly 30-fold after three years. We analyzed the spatial distribution of the disease in vineyards surveyed in 2020 and found that the disease distribution is highly aggregated based on Spatial Analysis by Distance Indices (SADIE). In a separate study, we also tested the GRBV infection status of asymptomatic vines next to symptomatic vines to inform decision-making when rogu-ing and replanting. Out of 410 asymptomatic vines surrounding 41 symptomatic vines, only two tested positive for GRBV. Additionally, in 2020 and 2021, we tested the GRBV status of previously identified possible alternative host species: blackberries (Rubus armeniacus) and wild/feral grapes (Vitis riparia) collected from areas surrounding the four survey sites. GRBV was present in 10 to 70% of wild grape samples in both years and in 10% of the blackberry samples in 2020. However, the virus titer was low in blackberry samples and it was not detected in 2021 samples. These results indicate the potential importance of wild grapes as alternative hosts on GRBD incidence and spread in southern Oregon vineyards, while blackberry is unlikely to be an alternative host with epidemiological significance.