Arboriculture & Urban Forestry 33(6): November 2007 377 Table 1. Diseases caused by Xylella fastidiosa. Disease Reference Alfalfa dwarf Almond leaf scorch Bacterial leaf scorch of landscape trees (BLS) Goheen et al. 1973 Mircetich et al.1976 (Table 2) Citrus variegated chlorosis Chang et al. 1993; Lee et al. 1993 Coffee leaf scorch Oleander leaf scorch Peach phony disease Pear leaf scorch Pecan leaf scorch Periwinkle wilt deLima 1998 Purcell et al. 1999c Hopkins et al. 1973 Leu and Seu 1993 Sanderlin and Heyderich-Alger 2000 McCoy et al. 1978 Pierce’s disease of grape Goheen et al. 1973; Hopkins and Mollenhauer 1973 Kitajima et al. 1975 Plum leaf scald uted, and sometimes treated, for various other biotic and abi- otic disorders. Although recognition of the pathogen is an important first step, there are many unanswered questions about X. fastidiosa and the diseases it causes in landscape tress. The economic consequences of X. fastidiosa in agronomic commodities such as grape, almond, citrus, and coffee are well recognized and provide significant impetus and support for research that indirectly benefits our understanding of BLS. Although some local and statewide surveys of BLS have been performed (Sherald et al. 1994; Sherald 2001; Lashomb et al. 2002), the compensatory and ecological losses caused by X. fastidiosa in the urban forest are only barely appreciated. A survey of oaks in several New Jersey com- munities found 30% to 35% of the highly susceptible oak species exhibiting symptoms. The economic impact of the disease was estimated to be between $0.7 and $1.6 million over the next 10 years (Gould et al. 2004). Hopefully, further studies like this will heighten the awareness of the signifi- cance of these diseases and encourage support for the research needed to answer the questions presented in this article. HOST RANGE Xylella fastidiosa has a wide host range, including over 150 species in 30 families of dicotyledonous and monocotyledon- ous plants (Freitag 1951; Raju et al. 1980; Raju et al. 1983; Hopkins and Adlerz 1988; Hill and Purcell 1995; McElrone et al. 1999; Hartman et al. 2001; Costa et al. 2004; Wistrom and Purcell, 2005; Shapland et al. 2006; XYLELLA FAS- TIDIOSA 2006). Currently, more than 40 tree species have been identified as hosts (Table 2). Although X. fastidiosa is difficult to isolate and grow in culture, diagnostic tools such as enzyme-linked immunosorbent assay (Sherald and Lei 1991), polymerase chain reaction (PCR) (Pooler and Hartung 1995), and immunomagnetic separation followed by PCR (Pooler et al. 1997) can detect X. fastidiosa in tissue extracts. Undoubtedly, additional hosts, including trees and other land- scape plants, will continue to be discovered using these and possibly other tools. Although many hosts are asymptomatic, they may still provide an alternative source of inoculum for tree infections. The high incidence of Pierce’s disease along the edges of vineyards has demonstrated that alternative hosts inhabiting the riparian edges are the primary source of inocu- lum (Hewitt et al. 1949; Goodwin and Purcell 1992). Hosts such as porcelain berry (Ampelopsis brevipedunculata), Asi- atic bittersweet (Celastrus orbiculatus), mugwort (Artemisia spp.), goldenrod (Solidago fistulosa), English ivy (Hedera helix), Virginia creeper (Parthenocissus quinquefolia), peri- winkle (Vinca major), blackberry (Rubus spp.), Bermuda grass (Cynodon dactylon), annual bluegrass (Poa annua), pe- rennial ryegrass (Lolium perenne), elderberry (Sambucus spp.), and wild grape (Vitis spp.) (Freitag 1951; Hopkins and Adlerz 1988; Sherald and Kostka 1992; McElrone et al. 1999; Hartman et al. 2001; XYLELLA FASTIDIOSA 2006) are common cultivated or wild-edge inhabitants in suburban and urban landscapes. These hosts may be significant factors in BLS epidemiology and may account for the prevalence of BLS in landscape settings rather than in forests. However, although X. fastidiosa may multiply in many hosts, not all hosts will support large bacterial populations nor will all hosts experience systemic spread of the pathogen (Hill and Purcell 1995; Purcell and Saunders 1999b; XYLELLA FAS- TIDIOSA 2006). Furthermore, their importance as alternative hosts also depends on their attractiveness to leafhoppers or other insect vectors. Therefore, it is unlikely that all alterna- tive hosts pose equal threats to tree populations. Unraveling the relationships among tree hosts, alternative host reservoirs, and insect vectors is a major challenge with significant bear- ing on BLS management. TRANSMISSION Xylem-feeding, sharpshooter leafhoppers in the subfamily Cicadellinae were known vectors of the causal agents respon- sible for Pierce’s disease, alfalfa dwarf, and phony peach disease long before X. fastidiosa was identified as the patho- gen (Hewitt et al. 1942; Houston et al. 1947; Turner 1949; Turner and Pollard 1959). Today, at least 24 species of leaf- hoppers are known vectors of Pierce’s disease (Frazier 1965) and 19 leafhopper species are vectors of alfalfa dwarf (Frazier 1944; Frazier and Freitag 1946). Four species of spittlebug, family Cercopidae, were found to be capable of transmitting X. fastidiosa in Pierce’s disease and alfalfa dwarf (Severin 1950). The glassy-winged sharpshooter (Homalodisca coagu- lata), GWSS, is one of the most important vectors. GWSS is endemic to the southeastern United States, where it is a major vector of X. fastidiosa in peach and grape. Since its arrival in California in the 1990s, it has become a serious threat to the California grape and almond industry and may become a threat to the citrus industry if the X. fastidiosa strains respon- ©2007 International Society of Arboriculture
November 2007
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