Arboriculture & Urban Forestry 33(6): November 2007 381 reduce leaf scorch symptoms in oak (The Bartlett Tree Re- search Laboratories, pers. comm.). PBZ’s growth regulator capacity may enhance the water efficiency of the plant, thus reducing symptom expression and the effect on the tree. Re- cent studies have also shown that PBZ at a rate of 20 g/ mL−1, ten times higher than the labeled rate, reduced in vitro growth of X. fastidiosa (DeStephano 2005). The effect on in vitro growth may be a consequence of the disruption of sterol biosynthesis (Sugavanam 1984; Burden et al. 1987; Raden- macher 1987). These studies have been largely exploratory so there is still considerable research that should be done in- volving a range of host and pathogen-affecting compounds and methods of application, including integrated and repeated treatments. The chronic nature of BLS provides an expanded window for a series of therapies that may ultimately be nec- essary to cure infected trees. Novel approaches such as bio- logical control may also hold promise. Recently, a benign strain of X. fastidiosa was found to be effective in providing induced resistance to Pierce’s disease (Hopkins 2005). The most lasting and practical approach to BLS manage- ment in landscape trees would be to find disease-tolerant selections. Tolerance has been found in grape, plum, and alfalfa and tolerance may exist in other hosts as well (Purcell 1979). Unfortunately, screening for tolerance is difficult be- cause of the fastidious nature of the pathogen, the slow and erratic nature of symptom development, and the difficulties associated with working with woody plants. Genetic engi- neering is being considered in the development of Pierce’s disease resistance and may also offer promise for landscape trees. The recognition of X. fastidiosa as a pathogen of landscape trees has been a significant advance over the last 25 years. Previously, leaf scorch and decline symptoms have been in- correctly attributed to a wide range of abiotic stressors and pathogens. Now that we are able to diagnose BLS with con- fidence, we must focus research on the difficult questions presented here, which are pertinent to controlling the spread and treatment of BLS. LITERATURE CITED Barnard, E.L., E.C. Ash, D.L. Hopkins, and R.J. McGovern. 1998. Distribution of Xylella fastidiosa in oaks in Florida and its association with growth decline in Quercus laevis. Plant Disease 82:569–572. Beale, J., P. Bachi, and J. Hartman. 2002. Landscape plant disease observations from the plant disease diagnostic laboratory—2002, pp. 22–23. In University of Kentucky Agricultural Experiment Station 2002 Nursery and Land- scape Program Research Report. www.ca.uky.edu/agc/ pubs/pr/pr468/pr468.pdf (accessed 8/1/2006). Bentz, J., and J.L. Sherald. 2001. Transmission of the xylem- limited bacterium Xylella fastidiosa to shade trees by in- sect vectors, pp. 203–208. In Ash, C. (Ed.). Shade Tree Wilt Diseases. American Phytopathological Society, St. Paul, MN. Blake, J.H. 1993. Distribution of Xylella fastidiosa in oak, maple, and sycamore in South Carolina. Plant Disease 77:1262. Burden, R.S., G.A. Carter, T. Clark, D.T. Cooke, S.J. Croker, A.H.B. Deas, P. Hedden, C.S. James, and J.R. Lenton. 1987. Comparative activity of the enantiomers of triadi- menol and paclobutrazol as inhibitors of fungal growth and plant sterol and gibberellin biosynthesis. Pesticide Science 21:253–267. Chagas, C.M., V. Rossetti, and M.J.G. Beretta. 1992. Elec- tron microscopy studies of a xylem-limited bacterium in sweet orange affected with citrus variegated chlorosis dis- ease in Brazil. Journal of Phytopathology 134:306–312. Chang, C.J., M. Garnier, L. Zreik, V. Rossetti, and J.M. Bove. 1993. Culture and serological detection of xylem- limited bacterium causing citrus variegated chlorosis and its identification as a strain of Xylella fastidiosa. Current Microbiology 27:137–142. Chang, C.J., and J.T. Walker. 1988. Bacterial leaf scorch of northern red oak: Isolation, cultivation, and pathogenicity of a xylem-limited bacterium. Plant Disease 72:730–733. Cochran, L.C., and L.M. Hutchins. 1974. Phony, pp. 96–103. In Virus Diseases and Noninfectious Disorders of Stone Fruits in North America. USDA Agricultural Handbook 437. Costa, H.S., E. Raetz, T.R. Pinckard, C. Gispert, R. Hernan- dez-Martinez, C.K. Dumenyo, and D.A. Cooksey. 2004. Plant hosts of Xylella fastidiosa in and near southern Cali- fornia vineyards. Plant Disease 88:1255–1261. Davis, M.J., A.H. Purcell, and S.V. Thomson. 1978. Pierce’s disease of grapevines: Isolation of the causal bacterium. Science 199:75–77. Davis, M.J., R.F. Whitcomb, and A.G. Gillaspie Jr. 1980. Fastidious bacteria of plant vascular tissue and inverte- brates (including so-called rickettsia-like bacteria), pp. 2172–2188. In Starr, M.P., H.O. Stolp, H.G. Truper, A. Balows, and H.G. Schlegel (Eds.). The Prokaryotes: A Handbook on Habitats, Isolation, and Identification of Bacteria. Springer-Verlag, New York/Heidelberg/Berlin. Day, M.F., and M.J. Bennetts. 1954. A Review of Problems of Specificity in Arthropod Vectors of Plant and Animal Viruses. C.S.I.R.O., Canberra, Australia. deLima, J.E.O., V.S. Miranda, J.S. Hartung, R.H. Brlansky, A. Coutinho, S.R. Roberto, and E.F. Carlos. 1998. Coffee leaf scorch bacterium: Axenic culture, pathogenicity, and comparison with Xylella fastidiosa of citrus. Plant Disease 82:94–97. DeStephano, D. 2005. Chemotherapeutic treatment options of Xylella fastidiosa in shade trees. M.S. Thesis, University of Maryland. https://drum.umd.edu/dspace/bitstream/ 1903/3251/1/umi-umd-3079.pdf (accessed 8/1/2006). ©2007 International Society of Arboriculture
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