Technical Abstract: The incidence of Pierce's disease caused by Xylella fastidiosa, was monitored in 11 naturally- infested commercial vineyards to examine the spatial patterns of the disease, elucidate possible influences of surrounding environments, and presence of a X. fastidiosa vector, Homalodisca coagulata (Say), (glassy-winged sharpshooter). Disease incidence ranged from <1% in field 1 in 2001 to 71% in field two in 2002. Disease incidence doubled in most vineyards during the 2002 production season. The lack of evidence for a gradient or presence of at most a weak gradient relative to GWSS sources (like citrus) was observed. Spatial patterns of symptomatic vines in 2001 and 2002, as determined by ordinary runs analysis, showed strong evidence for within and across-row aggregation of infected vines. Two-dimensional distance class analyses revealed a higher frequency of infected pairs of vines and a greater degree of clustering of diseased vines as disease incidence increased. In most fields, they were no disease gradients observed relative to GWSS source (e.g. citrus). Within fields, however, disease incidence displayed strong spatial dependence and a high degree of anisotropy, indicating strongly aggregated patterns of disease with distinct directional orientation. The within-row (0o) and across-row (90o) orientation generally were the predominant directions of increased disease incidence, consistent with vine-to-vine spread of X. fastidiosa. The predominant vector species observed within commercial vineyards was H. coagulata. Presumably, the distribution of PD in the field should reflect the feeding pattern of infectious vectors carrying the bacteria. Based on these results, effective PD management is likely to be based on practices that reduce significant insect vector populations, remove infected vines as soon as identified, and use of resistant cultivars.