380 Sherald: Bacterial Leaf Scorch of Landscape Trees have shown that bacteria occur in a slime-like matrix, or biofilm glycocalyx, which likely occludes tracheary elements (Mollenhauer and Hopkins 1974; Hearon et al. 1980; Chagas et al. 1992). Xylella fastidiosa is the first plant pathogen for which the complete genome has been sequenced, providing insight into the organism’s pathogenic potential (Simpson et al. 2000). Examination of the genome found that X. fastidiosa has the capacity to produce biofilm extracellular polysaccha- rides (EPSs). Biofilms are significant in the pathogenesis of many plant and mammalian pathogens enabling bacteria to attach to the host, colonize, and share enzymatic functions. Mutants lacking the ability to produce EPSs are usually non- pathogenic. Marques et al. (2002) have proposed that the biofilm is a likely virulence factor in X. fastidiosa. Intraplant movement is critical for pathogenicity. The ge- nome showed evidence of pectolytic enzymes capable of breaching the pit membranes between vessels. Genes were also found for encoding proteins necessary for the develop- ment of fimbriae or pili that are commonly observed on the polar ends of X. fastidiosa. Pili play an important role in bacterial colonization, enabling bacteria to attach to each other as well as to insect and plant hosts. Pili also provide twitching motility, which enables X. fastidiosa to move up- stream from the point of inoculation against acropetal sap flow (Meng et al. 2005). The mechanisms of pathogenesis may vary among hosts. The presence of genes encoding for the metabolism of iron and other metals suggests that some symptoms such as leaf variegation in CVC may be a consequence of the reduction of essential micronutrients in xylem fluid (Simpson et al. 2000). It is interesting that in phony peach disease, alfalfa dwarf, and ragweed stunt, X. fastidiosa simply causes a reduction in growth. It remains to be seen whether this response is solely a consequence of a reduction in water transport or some other mechanism. MANAGEMENT Currently, we do not know how to limit the spread of BLS or how to cure a BLS-infected tree. Typically, the first line of defense in plant disease management is sanitation, the re- moval of infected hosts. However, the chronic nature of BLS means that infected trees can remain aesthetically acceptable in the landscape for a long time before they must be removed. Because we do not even know if direct spread from tree to tree is a common occurrence, it is probably not advisable to engage in an aggressive sanitation program to remove in- fected trees when their removal may have little value in lim- iting disease spread. In Pierce’s disease, removal of infected grapes did not reduce spread (Hewitt et al. 1949). The host range for X. fastidiosa is extensive and far from completely known. Identification and removal of alternative host reservoirs may be an effective approach in reducing the incidence of BLS. However, the significance of alternative ©2007 International Society of Arboriculture reservoirs must first be confirmed. Some alternative hosts such as the invasive exotic species porcelain berry and Asi- atic bittersweet are extremely pervasive and some leafhop- pers can fly for long distances. Consequently, aggressive re- moval of these hosts over large areas will be difficult. An important research priority is to determine the signifi- cant insect BLS vectors. Understanding their life histories, including range of flight, preferred habitats, and hosts, is essential to prescribe management strategies. Because leaf- hoppers are generally active for long periods and X. fastidiosa persists in the adults, long-term canopy protection with in- secticides would be necessary. In the landscape, particularly in large municipal plantings, routine insecticide treatments are not an attractive or practical management approach. Man- agement of vector habitats and alternative hosts may be use- ful. Pierce’s disease occurs most commonly near riparian and other edge areas where leafhoppers overwinter and where alternative hosts occur (Hewitt et al. 1949; Purcell 1975; Goodwin and Purcell 1992). Recently, riparian vegetation management practices for Pierce’s disease have been devel- oped (Pierce’s Disease/Riparian Habitat Workgroup 2000). These practices prescribe the removal of host plants preferred by the blue–green sharpshooter for reproduction as well as the removal of systemic alternative hosts of X. fastidiosa, particularly invasive exotics. These species are replaced with native riparian plants that are not preferred by leafhopper or X. fastidiosa hosts. When symptoms first appear, if they are localized on a single branch, an attempt should be made to prune out the infection. The disease is chronic and in some hosts and indi- vidual trees, it may spread slowly, providing the opportunity to remove the infection before it becomes systemic through- out the tree. Because there is no way to know how far beyond the obvious symptoms the bacteria have gone, it is advisable to prune the branch as far beyond the visible symptoms as possible without destroying the aesthetic value of the tree. Studies to determine the spread of X. fastidiosa beyond ob- vious symptoms and the development of rapid techniques for tracing infections would be very helpful in directing thera- peutic pruning. Pruning is being used to manage CVC in Brazil. It has long been suspected that abiotic drought stress would exacerbate the effect of X. fastidiosa. McElrone et al. (2001) demonstrated that drought stress enhanced symptom progres- sion and severity in Virginia creeper (Parthenocissus quin- quefolia) artificially inoculated with X. fastidiosa. Horticul- tural practices that enhance root growth and minimize mois- ture stress may reduce symptom severity and prolong the life of infected trees. Chemotherapy with injected oxytetracycline has caused symptom remission but has not been effective in curing in- fected trees (Kostka et al. 1985; Chang and Walker 1988). The growth regulator paclobutrazole (PBZ) has been found to
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