STUDIES ON THE FUNDAMENTAL MECHANISMS OF XYLELLA FASTIDIOSA TRANSMISSION TO GRAPEVINES BY THE GLASSY-WINGED SHARPSHOOTER Project

The transmission of Xf to dormant grape by the glassy-winged sharpshooter (GWSS) was confirmed for a second year in outdoor cage trials in Bakersfield in January 2003, but at lower rates than in February 2002. Attempts to transmit Xf with GWSS from dormant grape and almond did not result in transmission. Sharpshooters fed on suspensions of Xf in xylem sap acquired Xf as determined by culture assays, but all failed to transmit Xf from suspensions to grape, suggesting that vector transmission requires a plant factor or environmental conditions not present in cultured cells. INTRODUCTION In attempts to reduce the economic impact of the glassy-winged sharpshooter (GWSS, Homalodisca coagulata), it is essential to better understand and evaluate GWSS's transmission of the pathogen Xylella fastidiosa (Xf). Xf has been causing diseases in California for a long time, but GWSS is apparently a more effective vector than other sharpshooters previously present in California in the field spread of Pierce's disease. We previously documented that GWSS transmits Xf with similar characteristics as other vectors (Purcell 2002). There is no or a very short (minutes) latent period; insects acquire and inoculate Xf with only 1 h of plant access period, although efficiency increases with larger access periods. Nymphs were as or more efficient vectors than adults, both for pathogen acquisition and inoculation, but lost infectivity after molting (Almeida and Purcell 2003). Overall, GWSS was at least 50% to 75% less efficient as a vector of Xf compared to the bluegreen sharpshooter (BGSS) (Purcell and Finlay 1979). One difference in the feeding behavior or GWSS compared to traditional California vectors such as BGSS was that adults transmitted Xf with similar efficiency to green tissues as to 2-year old wood of grapevines. Similar to previous results for BGSS (Hill and Purcell 1995), we found no correlation between the amount of the pathogen detection in the head of GWSS by culture and its transmission to plants. Our objectives for the past year were to further characterize Xf transmission by GWSS to grapevines and to develop methods by which GWSS could transmit Xf from artificial diets so as to be able to better control and experimentally manipulate bacterial acquisition. The final (#3) objective of the project was addressed without success the previous year; no bacterial isolates from GWSS proved to be taken up by GWSS from surface sprays of the bacterial isolates. OBJECTIVES 1. Characterize the transmission of Xf to grapes by GWSS. 2. Develop in vitro assays to assess vector transmission of Xf. 3. Test the possibility of biological control of Xf transmission through competition for attachment site in vector’s foregut. RESULTS GWSS transmission of Xf to dormant grapevines In a repeat of experiments conducted in an outdoor cage in Bakersfield in 2002, we confirmed that GWSS can transmit Xf to dormant grapevines, although rates were much lower than in 2002. We first confined GWSS on PD-grape source plants (strain STL) for 4 days in the greenhouse, then we caged groups of 4 adult GWSS per plant on grape seedlings. We transported 15 of these groups (60 insects total) on grape test plants and transferred one group per vine to a small mesh sleeve cage on field-grown Pinot Noir vines planted in an outdoor cage in Bakersfield, CA. We inoculated 15 field vines each on January 23 and 30, 2003 and removed the insects after one week. We diagnosed all plants after more than 3 months for PD symptoms and for Xf by culture assays (Hill and Purcell 1995) and/or by ELISA (Minsavage et al. 1994) in the CDFA Diagnostics Lab in Sacramento, using leaf petioles for diagnoses. Transmission rates in the greenhouse trials were 3 of 15 for January 19-23 inoculations and 4 of 15 for Jan. 26-30 inoculations in the lab on green vines. Only one of the 30 total groups of 4 GWSS transmitted to dormant grape in the field; this same group had also transmitted to a green grape test plant on which the insects were caged before exposure to field test plants. This rate of transmission (3%) was much lower than the 20-30% transmission achieved in identical experiments in February 2002. As we noted in our lab experiments over the previous three years, GWSS transmission to grape is quite variable among experiments (Almeida and Purcell 2003).