The topic of seminar this week was: "Xylella fastidiosa: pathogenicity, host specificity, and disease management." The talk was given by Don Hopkins, faculty member of the University of Florida Institute for Agricultural Sciences Mid-Florida Research & Education Center (UF/IFAS-MREC)
Xylella fastidiosa is a bacterium that used to be classified as fastidious, because it was considered to be unculturable. Currently, methods do exist for growing X. fastidiosa.
X. fastidiosa causes economic losses in grape (Pierce's disease), citrus (citrus variegated chlorosis, CVC), almond, coffee, peach, and plum. It is also responsible for decline of many urban shade trees and shrubs. There appear to be 3 subspecies with different host-specificities, which is what part of Dr. Hopkin's research is focused on. He isolated X. fastidiosa from different plant species, and inoculated other plant species with those isolates. He found that isolates were most pathogenic on the plant species they were isolated from, but some isolates could cause disease on several plant species.
The disease
Picture credit.The bacterium is spread by the glassy-winged sharpshooter, and infects the plant xylem, clogging it, and thereby slowing down the transpiration stream. The vascular obstruction causes symptoms of water stress. Bacterial toxins have also been proposed to be responsible for chlorosis and scorching symptoms, and growth regulators cold be the source of flattened dark green leaves, and shortened internodes which are among the symptoms of infected plants.
Research questions
Picture credit.One of the questions Dr. Hopkins was trying to answer was whether the virulence of the bacterium is related to the rate of colonization of the xylem vessels, or the movement of the bacterium within the xylem. He found that the ability of X. fastidiosa to colonize the plant systemically as opposed to staying locally at the site of initial infection, determined virulence and host specificity. Since the entire genome of Xylella fastidiosa has been sequenced, researchers can use the hints provided by research results to look for specific genes that are likely involved in pathogenicity and virulence.
Disease management
Because there both the bacterium and the insect vector have a wide host range, and there are a number of plant hosts that show little or no symptoms, it is difficult to sustain efforts to exclude either X. fastidiosa or the sharpshooter. Systemic insecticides, especially within the confines of a vineyard, can to some extent be used to control the insect vector, but is made more difficult by the fact that there are so many plant hosts. Elimination of inoculum sources is another strategy, and involves removal of infected trees.
One approach in the grape industry in California involves application of the soil-applied systemic insecticide Admire in May, monitoring the vineyards, and removal of infected trees.
Plant resistance is of little value in grape culture, where the genotypes of the crop are of immense importance to the quality of the wine. Transgenic resistance does show some promise, and currently there are some field trial underway with grape vines that have be genetically altered to include a lytic peptide gene.
Of major interest is current research that involves the use of a weekly virulent strain of X. fastidiosa obtained from elderberry, which appears to offer cross-protection in the field. Very young plants are inoculated with this strain early on with this mild strain. In one experiment, plants are still healthy more than 10 years after the initial inoculation. Further experiments to test this biological control agent are being conducted in several different states. Interestingly, the treatment is more effective if the initial inoculation is performed with a highly diluted bacterial suspension. The procedure is currently in the patent process. So far there is no data to support the idea that the sharpshooter spreads the biocontrol agent, but this is hard to test, because the protecting strain maintains very low numbers in the plant, and is often hard to detect.
Future research
Current and future experiments focus on the testing of the effectiveness of cross-protection on other grape genotypes, expansion of testing in commercial vineyards in different areas, and the use of X. fastidiosa strains to control other diseases. Data is being collected to submit to the EPA, commercial interest is being evaluated, and experiments are performed to get more data on the efficacy of the treatment.
Further reading
Almeida, R. No year. Xylella fastidiosa-A scientific and community internet resource on plant diseases caused by the bacterium Xylella fastidiosa. Online at: http://www.cnr.berkeley.edu/xylella/
Mizell, R.F., Andersen, P.C., Tipping, C. and Brodbeck, B. 2008. Xylella fastidiosa diseases and their leafhopper vector. Document ENY-683 (INA174) Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Online at: http://edis.ifas.ufl.edu/in174
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