IMPORTANCE OF GROUND VEGETATION IN THE DISPERSAL AND OVERWINTERING OF XYLELLA FASTIDIOSA Project Leaders:

The purpose of this project is to determine the ability of alternate host plants, specifically “weeds,” in almonds and vineyards to serve as reservoirs for Xylella fastidiosa (Xf) and for new inoculations by the glassy-winged sharpshooter (GWSS). We collected and analyzed weed and GWSS samples in and around commercial vineyard and almond fields for the presence of Xf on a monthly basis. Xf has been recovered from weeds collected during February and March, while no collected weeds tested positive for the presence of Xf between April and September. Monthly ground cover sampling will continue through the winter, as this time period may prove most important in the persistence of Xf over consecutive growing seasons. GWSS collected from alternate host plants have also been processed for Xf and have shown that adults collected on many species harbor Xf in their mouthparts. Results from these experiments will help to identify what time of year and what ground cover species are of most concern to growers wanting to control the spread of PD with minimal environmental impact. INTRODUCTION The economic viability of California’s vineyards and almonds has received considerable attention of late because of the expanding range of the glassy-winged sharpshooter (GWSS), Homalodisca coagulata , which can vector the xylem-limited bacterial pathogen, Xylella fastidiosa (Xf) (Goodwin & Purcell 1992, Redak et al. 2004). Xf is the causal agent of Pierce’s disease (PD) and almond leaf scorch (ALS) as well as other plant diseases. The arrival of GWSS has dramatically changed the epidemiology of Xf and its associated diseases in California (Redak et al. 2004). GWSS may not be an “efficient” vector of PD (Almeida & Purcell 2003a,b; Purcell & Saunders 1999a,b), but it presents a more serious threat, in part, because of its wide host range (Redak et al. 2004) and dispersal abilities (Blua et al. 2003). Of importance here is that the wide host range of Xf commonly overlaps with plant species visited by GWSS. Our proposed research will focus on the common host range of both vectors and pathogen, with an emphasis on potential annual weeds that may provide an overwintering reservoir for Xf and a spring feeding site for vectors of PD and ALS. How can this work impact control decisions? An excellent example of an overlooked insect-pathogen-host triangle is stinging nettle (Urtica urens), a common weed throughout the Central Valley. In our 2003 survey, we found that stinging nettle was a common host for GWSS in springtime, and recent DNA extraction showed the presence Xf in 60% of stinging nettle collected near a Kern County PD-infected vineyard. Whether or not Xf titer is high enough in these weeds for GWSS acquisition and transmission is not known, and is one aspect of the proposed study. Regardless, management of common hosts may be a critical component of epidemiology and area wide management of PD and ALS (Redak et al. 2004). With over 145 natural or experimental host plants for Xf that can cause PD, the insect/pathogen relationship is far too diverse a subject for one study. For this reason, we are studying the common landscape and ground vegetation found near vineyards and almonds in the San Joaquin Valley. Stinging nettle, Kern Co., 1/2004 Figure 1. Stinging nettle collected with the vine rows of a PD-infected vineyard showed 9 of 12 samples positive for Xf.