Spatio-Temporal Spread of Grapevine Leafroll Disease in Washington State Vineyards

The spread of grapevine leafroll disease (GLD) to vineyards planted with certified planting stock is of significant concern to grape growers. In this study, the spatial and temporal spread of GLD was examined in three vineyard blocks planted with virus-tested wine grape (Vitis vinifera) cultivars adjacent to vineyard blocks heavily infected with GLD in two geographic locations in eastern Washington State. During each season, the position of vines showing GLD symptoms was recorded in a matrix representing the planting lattice. Symptomatic vines were positive only for Grapevine leafroll-associated virus 3 (GLRaV-3), the most common virus species consistently associated with GLD in Washington vineyards. The results from multiple seasons showed a gradual increase in disease incidence over time in all three blocks. Spatial and temporal mapping of GLD indicated a disease gradient in which the highest percentage of symptomatic vines was in rows proximal to infected old blocks. Spatial autocorrelation analysis using Moran's I values suggested random patterns of symptomatic vines in the three blocks during initial years, indicating primary spread of the virus not related to infected vines within the block. Clustering at the scale of neighboring vines during subsequent years suggested secondary spread within the block. Results of quadrat-based spatial analyses of GLD incidence were compared with previously reported data obtained from California and elsewhere for an improved understanding of the dynamics of GLD spread to facilitate area-wide disease management strategies.