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

• Author(s): Purcell, Alexander;
• Abstract: Much remains to be discovered about the vector transmission of Xylella fastidiosa, and an understanding of this process may be essential in using methods that attempt to control X. fastidiosa diseases by reducing vector exposure to crops (insecticides, repellents, barriers, biological control). Information is needed to relate the numbers and activity of GWSS to disease spread in order to establish guidelines for vector control. A better understanding of the transmission process might provide new ideas for limiting disease spread by reducing X. fastidiosa transmission from plant to plant. Some of the important characteristics of transmission of X. fastidiosa by GWSS have not been described or estimated because so far there have been few studies of its transmission efficiency to grape or almond (Purcell and Saunders 1999). It has been suggested that the rapid impact of the GWSS in spreading Pierces disease (PD) may have been due to GWSS distinctive behavior of feeding on woody tissues and dormant plants rather than its abundance during the growing season. Transmission to woody tissues could increase the rate of chronic infection by X. fastidiosa if summer infections of the bases of grape canes or older wood establish chronic infections. Currently, it is thought that summer infections of the canes by traditional vectors in California do not often survive through the winter, explaining the lack of evidence to date for vineto- vine spread of X. fastidiosa in California vineyards (Purcell 1981, unpublished data). If GWSS can infect dormant vines or acquire X. fastidiosa from or transmit X. fastidiosa to dormant vines, this would extend the period during which vineyards should be protected from GWSS. Reducing the number of vectors does not necessarily reduce the amount of transmission of X. fastidiosa proportionally (Purcell 1981). We intend to fill research gaps of information on vector transmission efficiency and better understand how physical and biological factors affect transmission and perhaps how to interrupt vector transmission. The population levels of X. fastidiosa assessed by culturing bacteria from the head of the blue-green sharpshooter (BGSS) did not correlate with transmission (Hill and Purcell 1995). BGSS with levels of X. fastidiosa that were below a detection threshold of 100 cells per insect transmitted X. fastidiosa about as well as did BGSS with much higher populations of cultivable X. fastidiosa. The saturation of transmission efficiency by small numbers of X. fastidiosa implies that the active region in the vector from which the bacterium is transmitted is small. Understanding the efficiency and limitations of methods to detect X. fastidiosa is important, because estimates of what percentage of GWSS are capable of transmitting X. fastidiosa can otherwise only be made by transmission assays, which require special facilities and 6-10 weeks of incubation. The observations that (i) sharpshooter vectors stop transmitting X. fastidiosa after molting until they are again fed on an infected plant and (ii) there is no latent period between acquisition of X. fastidiosa from infected plants and its inoculation into healthy plants (Purcell and Finlay 1979a) imply that the bacteria are transmitted from the vectors foregut. Although X. fastidiosa has been observed in the foregut of vectors (Purcell et al. 1979c; Brlansky et al. 1983), the location of X. fastidiosa within the vectors foregut from which the bacterium is transmitted has not been proven. GWSS and other sharpshooters in the tribe Proconiini seem to transmit X. fastidiosa less efficiently than members of the Cicadellini. The two tribes of sharpshooters differ greatly in morphology and some behaviors, so it is possible that GWSS may differ significantly in behavior or morphology in ways that affect its transmission of X. fastidiosa. We seek to determine the location(s) of X. fastidiosa within the foregut that are critical for transmission. We hypothesize that X. fastidiosa may be transmitted in a similar manner to non-persistent viruses (Gray and Banerjee 1999), but with X. fastidiosa persisting by multiplication. Our efforts to identify the site within the vector foregut will test this hypothesis. We are planning with others (Backus, Walker, Blua, as described in other reports) to use electronic penetration graphics (EPG) to monitor the insects feeding activities, while identifying several phases of feeding such as ingestion and salivation and to identify critical phases of feeding during which inoculation of X. fastidiosa occurs. An in vitro assay for studies of the transmission of X. fastidiosa is essential to many related areas of research, such as tests of the transmissibility of mutants of X. fastidiosa and efficiency of transmission of different strains of X. fastidiosa. Such methods would allow experimental control over the acquisition of X. fastidiosa by the vector. Unfortunately, such a system has not been developed yet. Davis et al. (1978) tested the possibility of in vitro acquisition of X. fastidiosa, but the sharpshooters tested did not transmit. Purcell and Finlay (1979b) used the same approach to study sharpshooter transmission of bacteria other than Xylella. The development of an efficient in vitro system will enable experiments to determine what characteristics make X. fastidiosa vector transmissible. Understanding the transmission mechanisms of X. fastidiosa may also help develop biological control strategies for X. fastidiosa transmission. Microbes that occur on plant surfaces and can attach to the foregut surface of sharpshooters may compete with X. fastidiosa for a specific attachment site (or sites) in the GWSS foregut. This competition could exclude one of the microbes from this essential region in the vectors mouthparts, and the first microorganism to colonize it would in principle be the successful one. Our preliminary experimental data (unpublished) suggests that some GWSS are not able to transmit X. fastidiosa. Specifically; GWSS acquisition of X. fastidiosa does not asymptotically approach 100% as does the BGSS, but rather 10-70%, with high variability among different groups of GWSS. We seek to confirm and understand this phenomenon, including the possibility that other microbes can compete with X. fastidiosa for an attachment site on the GWSS foregut. We have isolated miscellaneous bacteria and yeast from the surface-sterilized heads of non-transmitting GWSS and from test plants fed upon by these insects in transmission tests. Our approach will be to look for reduced transmission after GWSS access to plants sprayed (or naturally infected) with different bacteria and fungi obtained from various sources (GWSS head, grapevine leaf surface and internal tissues).
• Publication Date: Dec 2001
• Journal: 2001 Pierce's Disease Research Symposium