Spatial Distribution of Pierce's Disease Related to Incidence, Vineyard Characteristics, and Surrounding Land Uses
A four-year (2001 to 2004) census was conducted in 220 vineyard blocks in the San Joaquin Valley (Kern County, CA) to characterize the spatial distribution patterns of Pierce's disease (PD) of grapevine caused by Xylella fastidiosa. Geostatistics and spatial analysis with distance indices were used to investigate relationships of PD incidence (i.e.; percentage of vines with PD) and occurrence (i.e., presence or absence of PD) with vineyard characteristics and the surrounding environment using a geographic information system. The census confirmed 52 vineyard blocks with at least one X. fastidiosa-infected grapevine and PD incidence in those vineyard blocks ranged from 0.001 to 29.8%. A total of 36 of the 52 blocks had fewer than five infected vines, so spatial analysis was not valid for these blocks. For the remaining 16 blocks, the distribution of X. fastidiosa-infected vines could be categorized as no structure, random, spatial trend, and aggregation as the incidence increased. Eleven blocks showed PD distributions that were consistent with primary X. fastidiosa spread, while two blocks showed aggregations that suggested secondary or vine-to-vine spread. The distribution in one block indicated a mixture of primary and secondary spread. Two blocks had recurring aggregations of X.fastidiosa-infected vines within rows that could be the result of vector feeding on adjacent plants within rows or bacterial spread on pruning implements. Significantly higher PD occurrence was found in Flame Seedless vineyard blocks, and other vineyard characteristics such as vineyard age, pruning method, and planting density were not related to PD occurrence. Although PD occurrence was not spatially related with surrounding environment, PD incidence was spatially related to citrus. This study provides spatial information concerning the epidemiology and sampling of PD in vineyards.