Relative to total sales, blueberries are the number one fruit commodity in the state of Georgia, surpassing even peaches. Production is concentrated in the southern coastal flatwoods. Rabbiteye blueberry (Vaccinium virgatum Aiton), a native species, has long been the predominant blueberry species cultivated in Georgia. More recently, however, growers have increased the production of the southern highbush cultivars (V. corymbosum interspecific hybrids) as a result of a very favorable market window (Scherm and Krewer, 2003). Several new diseases such as necrotic ringspot caused by tobacco ringspot virus and blueberry ringspot virus (Harald Scherm, personal communication) have appeared with the increased production of southern highbush cultivars. Recently, growers and scientists observed a new disorder affecting the southern highbush selection FL 86-19 in the Georgia blueberry production region. An initial symptom was marginal leaf scorch (burn) of the older leaves, similar to that observed with extreme drought or fertilizer salt burn. New developing shoots were usually abnormally thin with a reduced number of flower buds. Leaf drop eventually occurred with young twigs or stems of the southern highbush selection FL 86-19 developing a yellow, “skeleton-like” appearance. At this stage, the root system still appeared healthy, except for the possible loss of fine new roots. Whole plants or individual canes showed symptoms. The plant eventually died after leaf drop, typically during the second year of observation.
The previously mentioned symptoms were inconsistent with any previously reported blueberry disease. It was noticed, however, that the symptoms were similar to diseases caused by X. fastidiosa in other plants in south Georgia such as plum leaf scald (Raju et al., 1982; Wells et al., 1987) or pecan bacterial leaf scorch (Sanderlin and Heyderick-Alger, 2000). This observation prompted the initiation of testing to determine whether X. fastidiosa could cause these symptoms, and subsequent death, of blueberry plants.
Several reports in the past few years have revealed that X. fastidiosa is associated with diseases in a growing number of plant hosts (Almeida et al., 2005; Baumgartner and Warren, 2005; Hernandez-Martinez et al., 2007; Montero-Astua et al., 2008; Myers et al., 2007; Randall et al., 2007). No reports, however, specifically describe the aforementioned blueberry disease. Using a multiprimer polymerase chain reaction (PCR) system, random amplified polymorphic DNA–PCR, and sequence analysis of the 16S-23S rDNA intergenic spacer region, Hernandez-Martinez et al. (2007) reported the phylogenetic relationships of X. fastidiosa strains isolated from landscape ornamentals in southern California. They found that strains isolated from daylily, jacaranda, and magnolia clustered with members of X. fastidiosa subsp. sandyi and caused oleander leaf scorch but did not cause Pierce's disease on grapes. Strains isolated from Spanish broom and cherry and one strain isolated from western redbud clustered with X. fastidiosa subsp. fastidiosa members, whereas strains isolated from purple-leafed plum, olive, peach, plum, sweet gum, maidenhair tree, crape myrtle, and another western redbud strain clustered with members of X. fastidiosa subsp. multiplex. All the strains isolated from mulberry and one isolated from heavenly bamboo formed a separate cluster that has yet to be defined as a subspecies. In a separate experiment using nested PCR, Myers et al. (2007) identified the four most abundant sharpshooter leafhoppers in the eastern Piedmont region and the northeastern Coastal Plain in North Carolina: Oncometopia orbona, Graphocephala versuta, Parahlepsius irroratus, and Agolliota constricta. Among them, O. orbona, G. versuta, and P. irroratus tested positive for the presence of X. fastidiosa. Additionally, Myers et al. (2007) observed for the first time in the southeast that O. orbona and G. versuta were able to transmit X. fastidiosa to grapevines.
Almeida et al. (2005) showed that Homalodisca coagulata was able to inoculate X. fastidiosa into dormant grapes, although field acquisition experiments did not result in transmission. Montero-Astua et al. (2008) reported the finding of X. fastidiosa in avocado plants in Costa Rica based on the positive results of enzyme-linked immunosorbent assay (ELISA) and PCR, although attempts to cultivate the bacterium in periwinkle wilt (PW) medium (Davis et al., 1981) and buffered charcoal yeast extract medium (Feeley et al., 1979; Raju et al., 1982) were negative.
Furthermore, in their study of the persistence of X. fastidiosa in riparian hosts near north California vineyards, Baumgartner and Warren (2005) explained the higher Pierce's disease incidence in Napa vineyards relative to plants in Mendocino. This was based on their finding of higher X. fastidiosa populations in California grapes, Himalayan blackberries, and periwinkles in Napa and much less in the same three mentioned plants in Mendocino. Randall et al. (2007) concluded that X. fastidiosa is present in New Mexico and that the common landscape ornamental chitalpa is a host for X. fastidiosa based on positive results obtained from ELISA, PCR, and cultivation. At the time, direct pathogenicity testing was still in progress.
Based on ELISA and PCR techniques, it is clear that the association of X. fastidiosa with new hosts is increasing at a fast pace; however, most of the studies are awaiting fulfillment of Koch's postulates before the relationship between pathogen and host can be asserted. The research presented in this article provides clear evidence that X. fastidiosa is the causal bacterium for a new blueberry disease—herein named the bacterial leaf scorch of blueberry—by demonstrating the fulfillment of Koch's postulates.
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