The spread of Pierce's disease (PD) from riparian hosts to grapevines in California's north-coastal grape-growing region is a function of the proportion of Graphocephala atropunctata (blue-green sharpshooters [BGSSs]) that acquire Xylella fastidiosa from infected plant tissue. Riparian hosts that do not maintain sufficient X. fastidiosa populations for acquisition may not be significant inoculum reservoirs. We examined X. fastidiosa populations in systemically infected riparian hosts (California blackberry, California grapevine, elderberry, Himalayan blackberry, periwinkle) at two coastal locations (Mendocino and Napa) with two methods of quantitation (culturing and real-time polymerase chain reaction) from 2003 to 2004. In summer and autumn, X. fastidiosa populations were above the threshold for BGSS acquisition in periwinkle, Himalayan blackberry, and California grapevine at both locations. The only X. fastidiosa-positive plants detected in spring at both locations were periwinkle and Himalayan blackberry, suggesting that these species may contribute to long-term survival of X. fastidiosa. California blackberry and elderberry may not be important reservoirs of X. fastidiosa, given that very few plants of either species maintained infections. Higher X. fastidiosa populations in California grapevine, Himalayan blackberry, and periwinkle in Napa, relative to plants in Mendocino, may partially explain the higher PD incidence in Napa vineyards.