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The PI has interest in several insect-vectored disease projects at the present time, including aphid-vectored potyviruses in cucubits, whitefly-borne criniviruses in strawberry, and PD in grapes. Here we present objectives and procedures for PD, since it is one of the most pressing disease problems in California agriculture at the present time. It also serves as a model system and many of the techniques that are proposed for studying PD can be applied to other pathogen systems.
<ol> <LI> Determine the relationship between different densities of H. coagulata and X. fastidiosa primary spread and use this information to calculate an economic injury level.<LI>Determine the relationship between H. coagulata and X. fastidiosa secondary spread and use this information to evaluate grapevine roguing as a management tactic.<LI>Develop a comprehensive PD sampling program including vine-, areawide-, and vineyard-level PD sampling plans. <LI>Elucidate relationships between PD distribution and H. coagulata abundance.
NON-TECHNICAL SUMMARY: This project focuses on insect-vectored plant pathogens, with initial efforts aimed at studies on Pierce's disease (PD) of grapevines. This disease is caused by the bacterium Xylella fastidiosa and is vectored by sharpshooters. An introduction of the glassy-winged sharpshooter (GWSS), Homalodisca coagulata Say, in 1990 into California has resulted in severe epidemics of PD. Currently, we don't know very much about how GWSS moves X. fastidiosa into and within vineyards, and because of this lack of information it is difficult to design long-term management strategies for PD. This project will provide this basic information and allow us to establish economic thresholds for GWSS. In addition, we will develop a comprehensive sampling program based on the distribution of X. fastidiosa within grapevines, within vineyards, and within large geographic areas. With this sampling program, growers will be able to efficiently find infected vines within their vineyards so that actions can be taken to remove these vines.
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APPROACH: Introductions of new insects into agricultural areas can result in the spread of otherwise innocuous pathogens causing devastating results in certain commodities. Similarly, introductions of an exotic pathogen can lead to disease epidemics when they become transmitted by otherwise innocuous insects. Numerous examples of both exist, but perhaps the most significant in the last 15 years was the introduction of the glassy-winged sharpshooter (GWSS), Homalodisca coagulata Say, which vectored Xylella fastidiosa causing Pierce's disease (PD) in grapes in California. PD was identified in the Temecula Valley of Riverside County in 1997, and by 2002, the disease caused an estimated $13 million prompting the removal of approximately 40% of the vineyard acres. Currently, management strategies for PD attempt to limit both primary and secondary spread. Because the relationship between vector density and pathogen incidence is unknown, treatment thresholds for GWSS have been conservatively based on vector presence in citrus. If the relationship between GWSS density and X. fastidiosa incidence were known, the economic injury level could be calculated. In addition research to determine the PD incidence at which grapevine roguing will prevent pathogen spread is needed. A key component in the development of these management strategies is determining the location of infected vines within a vineyard, and the location of infected vineyards within a large geographic area. Without this knowledge, it is impossible to understand how X. fastidiosa is spread by GWSS and other vectors, or how features of the environment surrounding a vineyard (like other GWSS and PD hosts) impact movement of X. fastidiosa through the vineyard. Also, due to the limitations of the current symptom-based PD surveys, the majority of vineyards in California have never been surveyed for PD. Thus, for most of the state, we don't have any information on the extent of the PD epidemic. This project will develop PD sampling strategies that are not biased by putative symptomology, that are economical so they can be applied to large geographic areas, and that have sufficient detail so they can be used to locate diseased vines within vineyards. Our most recent studies have focused on elucidating the epidemiology of PD and developing a sampling program for PD detection. Epidemiological studies will determine the relative importance of primary and secondary spread which has direct application to management. The results of this work will be used to calculate the economic injury level for H. coagulata as a vector of X. fastidiosa, and to evaluate grapevine roguing as a management tactic to prevent X. fastidiosa spread. For the sampling program, we have identified 'PD-signature' areas in infected vineyards and the current project will investigate the use of remote sensing to locate these areas and prioritize vineyards for intensive sampling. This work will result in a comprehensive sampling program that utilized information gained at the vine, vineyard, and areawide levels.