Arboriculture & Urban Forestry 33(6): November 2007 379 mission. Successful transmission is driven by several factors, including their affinity for the host, seasonal host suscepti- bility, and the vector’s feeding habits (Day and Bennetts 1954; Turner and Pollard 1955; Purcell 1979). Determining the vectors is essential in understanding the epidemiology of BLS. Once vectors have been identified, factors such as their preferred habitat, alternative host preferences, and seasonal periods of transmission can be examined. Knowing the vec- tors and understanding their biology is fundamental in devis- ing disease management strategies. Xylella fastidiosa is a graft-transmissible, systemic patho- gen and caution should be exercised in the vegetative propa- gation of susceptible hosts. There is potential for root graft transmission; however, there is no epidemiological or direct evidence that root graft transmission of X. fastidiosa occurs in landscape trees. Root graft transmission should be investi- gated because the opportunities for root grafts are common in landscape settings. Natural root graft transmission of citrus variegated chlorosis has been demonstrated in potted sweet oranges (He et al. 2000). Because X. fastidiosa has been notoriously difficult to iso- late and grow in culture, it is assumed that there is little likelihood of it being mechanically transmitted in the course of routine pruning. However, until this avenue of transmis- sion is experimentally proven to be ineffective, caution is still warranted and pruning tools should be disinfected. PATHOGEN Xylella fastidiosa is a Gram-negative, rod-shaped bacterium that has specific nutritional requirements. Based on an analy- sis of 25 strains isolated from ten hosts, X. fastidiosa was described as a single species (Wells et al. 1987). However, genetic and phenotypic variation within the species has long been recognized and host-specific pathotypes have been iden- tified. For example, strains shown to cause Pierce’s disease of grape also cause almond leaf scorch and alfalfa dwarf (Mir- cetich et al. 1976; Davis et al. 1980). Similarly, the strain or strains responsible for phony peach disease are also respon- sible for plum leaf scald (Wells et al. 1981). Strains causing citrus variegated chlorosis (CVC) are closely related to strains responsible for coffee leaf scorch (CLS) and can cause CLS symptoms when inoculated into coffee (Li et al. 2001). Strain relationships become more complex when genetically distinct strains cause similar symptoms in the same host. Strains responsible for CVC and CLS are readily distinguish- able from Pierce’s disease strains yet can cause Pierce’s dis- ease symptoms in grape, whereas Pierce’s disease strains do not cause symptoms in citrus (Li et al. 2002). Reciprocal transmission studies involving shade trees have only been performed with isolates from elm and sycamore and in this study, symptoms could only be produced in the species from which the isolate was obtained (Sherald 1993). Recently, three subspecies of X. fastidiosa have been de- scribed: two North American subspecies, X. fastidiosa subsp. piercei and X. fastidiosa subsp. multiplex, and a South Ameri- can subspecies, X. fastidiosa subsp. pauca (Schaad et al. 2004). A fourth subspecies, X. fastidiosa subsp. sandyi, has also been proposed for oleander leaf scorch isolates (Schuen- zel et al. 2005). Subspecies classification of X. fastidiosa strains responsible for BLS is far from resolved. Oak leaf scorch strains have been placed in X. fastidiosa subsp. mul- tiplex (Schuenzel et al. 2005). Schaad et al. (2004) have placed a maple strain in X. fastidiosa subsp. piercei and sy- camore and elm strains in X. fastidiosa subsp. multiplex. Al- though strains may be readily distinguished by molecular and genetic techniques, host response is the most meaningful test of strain significance and the hardest to demonstrate. The relationships of strains to hosts, both primary and alternative, as well as to insect vectors are numerous, complex, and very difficult to unravel. Understanding these relationships may have important disease management implications. Survival and growth of X. fastidiosa are likely dependent on several factors, including temperature. Studies of Pierce’s disease strains have shown that rapid growth occurs between 25 and 32°C (77°F and 89.6°F). Temperatures below 12°C to 17°C (53.6°F to 62.6°F) and above 34°C (93.2°F) adversely affect survival of the pathogen (Feil and Purcell 2001). The significance of warm temperatures is demonstrated by the fact that symptoms of Pierce’s disease appear earlier in warmer areas and expand beyond their normal range after mild winters (Feil and Purcell 2001). Temperature optimums have not been determined for strains associated with land- scape trees. Occurrences of BLS in Kentucky, Pennsylvania, New Jersey, New York, and southern Ontario suggest a greater tolerance of colder temperatures. However, like Pierce’s disease and the other diseases caused by X. fastidi- osa, BLS occurs predominantly in the southern and mid- Atlantic states where winters are mild. As global warming increases, BLS may expand further north and the incidence and severity of these diseases could increase throughout their range. This would be a consequence of several factors, in- cluding an increase in the survival and growth of the patho- gen, an increase in the growing season and period of suscep- tibility of the host, and an increase in heat and moisture stress. PATHOGENESIS The process by which X. fastidiosa causes disease is not completely understood. Phytotoxins, growth regulator imbal- ance, and water stress have all been considered possible mechanisms of pathogenesis (Hopkins 1989). Physiological studies have shown a relationship between occluded vessels and symptom development in grape (Hopkins et al. 1977, 1981). In elm, Kostka et al. (1986a) showed a reduction in stem hydraulic conductivity and stem water potential in in- fected elms. Numerous observations of X. fastidiosa in planta ©2007 International Society of Arboriculture
November 2007
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