Spring qPCR Provides Early Detection of Grapevine Red Blotch Virus Infection in Cabernet Franc Sentinel Vines

Although knowledge of grapevine red blotch virus (GRBV) biology has grown substantially, uncertainty remains regarding the optimal timing of qPCR testing to detect newly positive vines before they contribute to secondary spread. Using 400 Cabernet franc sentinel vines planted in a vineyard with high GRBV incidence, we evaluated how early new positive vines can be detected by qPCR using petiole tissue and whether spring testing captures most new positives observed within a growing season. A single sentinel vine tested positive one year after planting. Across 2022-2025, nearly all newly positive vines were detected in spring, with few additional detections in summer or fall. Spring-positive vines exhibited high, uniformly distributed virus titers but remained asymptomatic for four months. These findings indicate that spring qPCR testing reliably identifies most newly GRBV-positive vines within a growing season and may enable timely removal of infected vines before peak Spissistilus festinus activity, thereby potentially reducing secondary spread.