Bacteria exhibit an optimal growth rate in culture media with sufficient nutrients at an optimal temperature and pH. In addition, the concentration of solutes plays a critical role in bacterial growth and survival. Glutamate is known to be a major anionic solute involved in osmoregu-lation and the bacterial cell's response to changes in solute concentration. To determine how glutamate uptake is involved in osmoregulation in the rice bacterial pathogen Burkhol-deria glumae BGR1, we mutated the gltI gene encoding a periplasmic substrate binding protein of a glutamate transport system to abolish glutamate uptake, and monitored the growth of the gltI null mutant in Luria-Bertani medium. We found that the gltI null mutant showed a slower growth rate than the wild-type strain and experienced hyperosmotic stress resulting in water loss from the cytoplasm in stationary phase. When the incubation time was extended , the mutant population collapsed due to the hyperosmotic stress. The gltI null mutant exhibited loss of adaptability under both hypoosmotic and hyperosmotic stresses. The growth rate of the gltI null mutant was restored to the level of wild-type growth by exogenous addition of glycine betaine to the culture medium, indicating that glycine betaine is a compatible solute in B. glumae. These results indicate that glutamate uptake from the environment plays a key role in osmoregulation in B. glumae.

Burkholderia glumae is the primary causal agent of bacterial panicle blight of rice. In this study, 11 naturally avirulent and nine virulent strains of B. glumae native to the southern United States were characterized in terms of virulence in rice and onion, toxofalvin production, antifungal activity, pigmentation and genomic structure. Virulence of B. glumae strains on rice panicles was highly correlated to virulence on onion bulb scales, suggesting that onion bulb can be a convenient alternative host system to efficiently determine the virulence of B. glumae strains. Production of toxoflavin, the phytotoxin that functions as a major virulence factor, was closely associated with the virulence phenotypes of B. glumae strains in rice. Some strains of B. glumae showed various levels of antifungal activity against Rhizoctonia solani, the causal agent of sheath blight, and pigmentation phenotypes on casamino acid-peptone-glucose (CPG) agar plates regardless of their virulence traits. Purple and yellow-green pigments were partially purified from a pigmenting strain of B. glumae, 411gr-6, and the purple pigment fraction showed a strong antifungal activity against Collectotrichum orbiculare. Genetic variations were detected among the B. glumae strains from DNA fingerprinting analyses by repetitive element sequence-based PCR (rep-PCR) for BOX-A1R-based repetitive extragenic palindromic (BOX) or enterobacterial repetitive intergenic consensus (ERIC) sequences of bacteria; and close genetic relatedness among virulent but pigment-deficient strains were revealed by clustering analyses of DNA fingerprints from BOX-and ERIC-PCR.

Pecan (Carya illinoinensis) bacterial leaf scorch disease, caused by the bacterium Xylella fastidiosa, causes leaf loss and reduction in yield of pecans. One of the ways that the pathogen infects newly developing trees is by graft-transmission through infected scion wood. Submersion of pecan scion wood in 46°C water for 30 min greatly reduced transmission of X. fastidiosa following grafting. During a 2-year study with potted rootstock grafted to either hot-water-treated or nontreated scion wood collected from limbs of ‘Cape Fear’ pecan infected with X. fastidiosa, the pathogen was detected in 21% of the trees that developed from the nontreated scion wood and 0.7% of the trees from the hot-water-treated scions. The hot-water treatment of 46°C for 30 min did not affect graft success. Likewise, scion diameter had no effect on success of grafting or on the efficacy of hot-water treatment. Similar hot-water treatments have been efficacious in elimination of X. fastidiosa and some other pathogens from grapevine cuttings. The use of the hot-water treatment demonstrated in this report could be helpful to individual pecan growers and nurseries that use scion wood that may be infected with X. fastidiosa to reduce the occurrence of pecan bacterial leaf scorch disease in new trees.

Me!anson, R. A., Sanderün. R. S., McTaggart, A. R., and Ham, J. H. 20! 2. A systematic study revea!s that Xyiella fastidiosa strains from pecan are part of X, fastidiosa subsp. multiplex, P!ant Dis. 96: ! 123-! 134.Xyiella fastidiosa causes disease in a number of economicaüy important crops, ornamenta! p!ants, and shade trees, inc!uding grapevine, citrus, oteander, and sycamore. In pecan, X, fastidiosa causes pecan bacteria! !eaf scorch (PBLS), whic!i !eads to defoüation and reduces nut yield. No economically effective treatments are availab!e for PBLS. !n order to improve PBLS management practices, it is necessary to determine tbe subspecies of X, fastidiosa strains that infect pecan so that potentia! sources of inocu!um may be identified. Mu!tiprimer polymerase chain reaction (PCR) and phy!ogenetic ana!yses using nuc!eotide sequence data from the 16S-23S rRNA intergenie tran-scribed spacer (ITS) region and pglA consistently identified strains of X, fastidiosa isolated from pecan as X, fastidiosa subsp. multiplex, Enterobacteria! repetitive intergenie consensus PCR and repetitive extragenic paündromic (REP)-PCR ana!yses were congruent with phy!ogenetic ana!yses. REP-PCR ana!yses indicated genetic variation within strains of X, fastidiosa from pecan. From these same analyses, X, fastidiosa strains from sycamore, grapevine, and oleander from Louisiana were identified as subsp. multiplex, subsp. fastidiosa, and subsp. sandyi, respective!y. This study provides additiona! information about the host ranges of X. fastidiosa subspecies.

Pecan bacterial leaf scorch (PBLS), caused by the bacterium Xylella fastidiosa, can cause economically significant crop loss to some pecan (Carya illinoinensis) cultivars in the southeastern United States. X. fastidiosa is typically vectored by spittlebugs (Cercopidae) and leafhoppers (Cicadellidae). Because no vector species had been reported for pecan, an attempt was made to identify potential vectors that are capable of acquiring the bacterium from infected pecan trees and transmitting to pecan. Several spittlebug and leafhopper species collected from various sources, including sorghum and pecan, were tested as potential vectors of the pathogen from pecan to pecan. When tested in groups, the pecan spittlebug, Clastoptera achatina; the Johnson-grass sharpshooter, Homalodisca insolita; and the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis, had transmission rates to pecan of 11.4, 19.3, and 4%, respectively, following a pathogen acquisition period on infected pecan terminals. The pecan spittlebug is common in pecan orchards in the southeastern United States, and the GWSS was observed on young vigorous pecan shoots. Limited testing with the diamond-backed spittlebug, Lepyronia quadrangularis, and the lateral-lined sharpshooter, Cuerna costalis, suggested that these could be occasional vectors of X. fastidiosa to pecan. There is a need for studies on the identification and population dynamics of Cicadellidae that inhabit pecan orchards to determine if management of vectors is needed in commercial pecan production to reduce the spread of PBLS.