Seasonal variation in grapevine red blotch virus titer in relation to disease symptom expression in vineyards

Grapevine red blotch virus (GRBV) is the causative agent of red blotch disease. Limited information is available on the seasonal variation of GRBV titer in relation to disease symptom expression in vineyards across the United States. In this study, no statistically significant difference in GRBV titer was found among asymptomatic, infected vines in June (p=0.451) and among symptomatic, infected vines in October (p=0.068) in a diseased ‘Cabernet franc’ vineyard in California, regardless of the years symptomatic, i.e., one to seven, as shown by qPCR. Similarly, there were no statistically significant differences in GRBV titer as it relates to isolates of the two phylogenetic clades in asymptomatic, infected ‘Cabernet franc’ and ‘Cabernet Sauvignon’ vines in June (p=0.138 and p=0.778, respectively) and in symptomatic, infected vines in October (p=0.806 and p=0.490, respectively). GRBV titer differed among cultivars in diseased California vineyards (p < 0.001) and increased over the course of the growing season in infected ‘Merlot’ and ‘Cabernet franc’ vines, but not in ‘Cabernet Sauvignon’ vines. Patterns observed in California were consistent in New York and Georgia vineyards. In a Geneva double curtain-trellised ‘Cabernet Sauvignon’ vineyard in Georgia, GRBV distribution was uneven between cordons, and virus titer was variable within the vine canopy in June (p=0.017) but not in October (p=0.107). This work revealed consistent patterns of GRBV titer during a growing season in different vineyards across the United States. It also highlighted relatively high virus titer in symptomless grapevines in June when Spissistilus festinus-mediated GRBV transmission is documented in northern California.