Xylella fastiodiosa -- Bacterium
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Pierce’s Disease is caused by a xylem-dwelling bacterium, Xyella fastiodiosa. These bacteria physically clog the xylem and xanthum gum production that increases water stress in greapes and other susceptible plants. The plants show scorch-like symptoms before dying of dehydration. The bacterium is vectored by the Glassy-winged sharpshooter leafhopper which invaded California from Florida. This insect will probably spread the bacterium to the Northern California premier wine districts. The leafhopper is polyphagous and feeds on a wide variety of ornamental, agricultural and native plants.
According to Dr. Mark S. Hoddle of the University of California, classical biological control of the leafhopper may be the most effective method of suppressing the threat (Hoddle 2002) Mymarid egg parasitoids in the genus Gonatoceras are considered promising candidates for introduction. Two species, G. iriguttatus (from Texas) and G. fasciatus (from Louisiana) have thus far been liberated for establishment. Additional species are being sought to curb this extremely important threat to the wine industry. However, successful classical biological control of a vector insect such as the Glassy-winged sharpshooter is not very probable because only a single individual insect can transmit the bacterium. To reduce the vector populations to a very low level with a group of insects such as leafhoppers has never been accomplished in similar situations (e.g., beet curly top vectors in the American West). Yet, some seemingly unlikely candidates for biological control have been reduced successfully by continuous effort in searching for effective natural enemies (e.g., navel orangeworm on almonds by bethylid parasitoids).
By the year 2008 it has become evident that only certain plant species are susceptible to the bacterium. For example, among the oleanders, only the white-flowering varieties succumb to the bacterium: the pinks, reds, magentas and other varieties that bear colored blossoms are immune. This raises the possibility that in the grapes a similar resistance may be observed. Nevertheless, a close scrutiny of the problem as it develops is essential.
REFERENCES: [Additional references may be found at: MELVYL Library ]
Hoddle, M. S. 2002. Applied Biological Control. The Buzz, UC Riverside, Dept. of Entomology Newsletter. 1-2.
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