Baumgartner, K, S Greenleaf, JF Quinn, and JH Viers. 2006. Significance of Riparian Plants in the epidemiology of Pierce’s Disease. Submitted to California Department of Food and Agriculture Pierce’s Disease and Glassy-wing Sharpshooter Board. 4 ppd.

We examined the relationship between the occurrence of Pierce’s disease (PD) in Napa Valley vineyards and both adjacent and distant vegetation types.  Because the vector, Graphocephala atropunctata (blue-green sharpshooter, BGSS), is mobile and has a broad host range, disease risk is influenced by vector migration among vegetation types.  Therefore, certain combinations of vegetation types surrounding vineyards are more likely to be associated with PD. To test this hypothesis, we surveyed for PD in a total of 41 vineyards located adjacent to either riparian woodland (vector habitat), urban land (vector habitat), other vineyards (vector habitat), or oak woodland (habitat status unknown).  The proportions of the four vegetation types distant from the sites (within 0.5, 1, 1.5, and 2 km) were quantified with a geographical information system.  Vineyards were surveyed for PD in 11/05.  Pathogen presence was confirmed by ELISA.  Multiple binary logistic regression showed that both adjacent and distant vegetation type significantly predicted PD presence.  Vineyards were more likely to have PD if they were adjacent to riparian woodland and surrounded by more vineyards or urban land.  These results suggest that vineyards and urban lands may be important in PD epidemiology.  Given that uninfected vineyards adjacent to riparian woodland were also surrounded by large amounts of riparian and upland woodland, it is also possible that riparian woodland in more forested landscapes hosts lower vector densities or a lower proportion of infective vectors.  Alternatively, more expansive woodland may be associated with lower PD risk because it decreases the spread of infective BGSSs.