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United States Department of Agriculture

Agricultural Research Service



2009 Annual Report

1a. Objectives (from AD-416)
Objective 1: Define the distribution patterns of Xylella fastidiosa and Candidatus Liberibacter spp. in planta. Objective 2: Characterize host/pathogen interactions and potential control methods for Candidatus Liberibacter spp. and Citrus. Objective 3: Characterize antagonistic or synergistic interactions between strains of Xylella fastidiosa from sweet orange and grapevine. Objective 4: Establish the etiology of citrus chlorotic dwarf disease. Objective 5: Characterize, using microassay hybridizations and other approaches, host plant gene expression pattern in response to infections by 'Candidatus Liberbacter' spp. (citrus greening pathogens).

1b. Approach (from AD-416)
We will characterize novel or exotic pathogens of citrus that threaten the U.S. citrus industry. This work will include the determination of the etiology of novel diseases, characterization of the diversity present in a pathogenic taxon, the development of diagnostic tests for the pathogen, and the development of information about disease processes and knowledge that may lead to control of the diseases. This project will also maintain and develop an extensive collection of exotic pathogens of citrus for use by this project and collaborators from the U.S. and around the world. Detection methods will e generally PCR-based, including real-time quantitative PCR. Antibodies will also be developed. The etiology of novel diseases will be established by plant inocolulations both in the greenhouse in Beltsville and by field experiments in the countries where the disease exists. We will characterize responses of diseased plants to infections by citrus greening pathogens through the use of microarray analyses and other nucleic acid-based approaches and methodologies to identify genes whose expression is altered in infected plants compared to disease-free plants.

3. Progress Report
A quantitative PCR assay for the citrus greening pathogen previously developed by this project was used in several research activities. The assay was applied to detect the citrus greening pathogen in seedlings germinated from seed collected from trees and fruit with symptoms of citrus greening disease. From our results it appears that the citrus greening pathogen cannot be transmitted through true seed from an infected tree to seedlings. The quantitative PCR assay for the citrus greening pathogen was also used to characterize the transmission of the pathogen between plants of different types by using the parasitic dodder plants. Dodder plants are interesting because their ‘roots’ can be made to grow into a greening infected plant and its shoot can be made to produce new ‘roots’ on a healthy plant of virtually any species. By this means the host range of pathogens can be determined. We have extensively and quantitatively characterized this process using dodder plants. As we showed previously with infected citrus, the distribution of the citrus greening pathogen in dodder is not uniform, but the pathogen clearly multiplies to high levels in the dodder plant. We have also used dodder plants in efforts to transmit the citrus chlorotic dwarf virus (CCDV) to a special variety of tobacco which would be a more amenable host for experiments than is citrus. Cross species grafts from CCDV infected citrus to tobacco have also been successfully completed. We have initiated work on a new objective to characterize changes in gene expression in citrus in response to infection with the citrus greening pathogen. Experimental plants and methods of nucleic acid extraction and analysis have been established. We have also begun work on a project funded by outside funds to develop antibodies against the citrus greening pathogen. This includes recruiting additional staff and setting up several cooperative research agreements.

4. Accomplishments

Review Publications
Li, W.N., Levy, L., Hartung, J.S. 2009. Quantitative Distribution of Candidatus Liberibacter asiaticus in Citrus Plants and Fruits Infected by Citrus Huanglongbing. Phytopathology. 99:139-144.

Lacava, P.T., Azevedo, J.L., Miller, T., Hartung, J.S. 2009. Interactions of citrus variegated chlorosis (CVC) with endophytic bacteria. Tree and Forestry Science and Biotechnology. 3:40-48.

Last Modified: 2/23/2016
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