Spatial distribution of Pierce's disease in the Coachella Valley: Implications for sampling
A three-year survey was conducted to characterize the spatial distribution patterns of Pierce's disease (PD) of grapevines in the Coachella Valley (Riverside County, CA). Spatial structures of PD distributions were characterized with geostatistics and spatial analysis with distance indices (SADIE). The survey confirmed 97 diseased grapevines from seven vineyards in the valley, and the diseased vines were localized within the vineyards. Approximately 82% of diseased grapevines were found near two to six consecutive dead, missing, or replanted grapevines in a row. Six vineyards had low (< 0.01%) PD incidence, but one vineyard had 3.8% incidence and the diseased vines were spatially aggregated, forming a significant (p < 0.05) patch. The diseased grapevines within the patch also were aggregated and spatially correlated within 26 m. This is the first report of PD distribution in the Coachella Valley, and these findings provide the foundation for development of PD sampling plans.