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United States Department of Agriculture

Agricultural Research Service


Location: Subtropical Plant Pathology Research

2009 Annual Report

1a. Objectives (from AD-416)
1) Identify genetic diversity of HLB pathogen(s) in China and the United States. 2) Identify molecular and biological mechanisms of citrus HLB and control of the disease

1b. Approach (from AD-416)
Identify and characterize the disease-free citrus plant (s) from historically and heavily HLB-affected orchard (over 10-15 year old citrus plants). Isolate and identify the microbial consortium from the rhizosphere and endophytes from the identified disease-free plants by culture the microbes and sequencing the microbes using molecular markers. Characterize the role of Can. L asiaticus in the HLB disease development using functional genomics. Characterize the host gene expression and regulation of the citrus plants those are resistant or tolerant to HLB by using microarray and real-time PCR. Characterize the biological and molecular mechanisms of HLB bacterial transmission by the Asian Citrus psyllid using comparative and functional genomics. Explore and apply the biocontrol potentials of the psyllids by using biocontrol microorganism and parasitic or predator insects. Screening chemicals against HLB and application of the selected chemical in the fields and monitoring the dynamics of the disease in the field.

3. Progress Report
This project relates to objective 1 of the inhouse project: Characterize ecology, biology, epidemiology, genetics and host interactions of domestic, exotic, newly emergent and re-emerging pathogens. The objective of this project is 1) to reveal the genetic diversity of Huanglongbing (HLB) pathogens in China and the United States, and 2) to study the microbial ecology of HLB-infected citrus plants and its insect vector, the Asian citrus psyllids. Based on the genome sequence of Candidatus Liberibacter asiaticus (Las) that we obtained, we designed new primers and probes that target various regions of the bacterial genome. Using these newly developed primers and probes along with standardized protocols, we revealed the genetic diversity of Las bacteria collected from Florida and China. The results indicated that the HLB pathogen in Florida is different from that in China, suggesting the Las bacterium was not introduced from China. More samples of Las-infected citrus and psyllids are being collected from different geographic locations and different citrus varieties from China. These samples will be subjected for the analysis of genetic diversity of the pathogens and microbial diversity of the infected host plant and insect using the standardized protocols. Progress was monitored through repeated phone calls and email.

4. Accomplishments

Last Modified: 2/23/2016
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