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Submitted to: Symposium Proceedings
Publication Type: Proceedings Publication Acceptance Date: 6/22/2007 Publication Date: 7/18/2007 Citation: Backus, E.A. 2007. How to be an ideal vector: Four crucial steps in the transmission mechanism of Xylella fastidiosa by sharpshooters. In: Proceedings of the National Viticulture Research Conference, July 18-20, 2007, Davis, CA. p. 9-10. Interpretive Summary: Recent studies of the glassy-winged sharpshooter, Homalodisca vitripennis, are beginning to show how feeding controls inoculation (expulsion) of Xylella fastidiosa into plants. To date, work supports a model of four crucial steps that must all occur for successful injection of the bacteria into a plant. Evidence for each of these is briefly summarized, and results from a recent test of the model are described in detail. Sharpshooters were allowed to acquire X. fastidiosa, then to make a controlled, standardized feed on a healthy grape plant. Test plants all developed symptoms of X. fastidiosa infection in 2 – 3 months, most were PCR-positive for X. fastidiosa, and all died at 5 months. Thus, when experimental conditions are manipulated to guarantee each of the model’s four steps at once, a single, individual sharpshooter can be an “ideal vector” (i.e. 100% likelihood of inoculation by one feed on a susceptible grape). This and related work will identify the precise means by which X. fastidiosa is inoculated, and provide needed information for development of resistant grape varieties, microbial biological control, and epidemiological modeling of disease progression. Technical Abstract: This paper discusses recent research on how the feeding of glassy-winged sharpshooter, Homalodisca vitripennis controls transmission (acquisition and inoculation) of Xylella fastidiosa. Studies to date support a model of four crucial steps that must all occur simultaneously during H. vitripennis feeding, for successful inoculation. These are: 1) the titer of acquired bacteria (bacterial load) present in the vector foregut must be high, 2) bacteria must be present in a precise location in the foregut, probably the anterior precibarium, 3) the vector must perform a specific inoculation behavior, comprising both egestion and salivation, and 4) the vector’s stylet tips must be in a functional xylem cell at the time of inoculation. Evidence for each of these is briefly summarized, then results from a recent experiment are described in detail. Sharpshooters were caged on infected grape for 7 – 8 days to acquire X. fastidiosa, then allowed to make a single, standardized electronically monitored probe on a healthy grape plant. Test plants all became symptomatic of X. fastidiosa infection in 2 – 3 months, and all died at 5 months. Non-optimized PCR tests performed at 3 months showed most plants were Xylella-positive. Thus, when experimental conditions are manipulated to maximize each of the model’s four steps at once, a single, individual sharpshooter can be an “ideal vector” (i.e. 100% inoculation efficiency within one probe on susceptible grape). This and related work will identify the precise mechanism of X. fastidiosa transmission, with implications for development of resistant grape varieties, microbial biological control, and epidemiological modeling of disease progression. |
