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United States Department of Agriculture

Agricultural Research Service


Location: Subtropical Plant Pathology Research

2010 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 research relates to inhouse objective 1: Characterize ecology, biology, epidemiology, genetics and host interactions of domestic, exotic, newly emergent and re-emerging pathogens. The project is currently at the second year. The following is a summary of the research results. 1) Genetic diversity of the HLB pathogens in China and the United State Samples from various HLB-infected citrus varieties were collected both from China and the United States, and the DNAs were isolated and subjected to genetic diversity studies based on the hyper-variable regions of the HLB bacterium, ‘Candidatus Liberibacter asiaticus’(Las). Sequence analysis of the hyp1 and hyp2 genes of these isolates indicated their genetic diversity between these two origins, which suggested the Las strain in Florida is different from those in China. 2) Microbial ecology of the HLB-infected citrus plants and its insect vectors, the Asian citrus psyllids Samples from three different varieties (Citrus tangerine, C. sinensis, C. reticulata) of citrus plants with and without HLB disease were collected and subjected to microbial ecology studies using phospholipid fatty acid (PLFA) analysis. Different PLFA biomarkers were revealed from HLB-infected vs. non-infected plants. These markers will be further evaluated against broader host ranges (varieties) and number of samples to identify useful markers for diagnostics. Microbial community dynamics in the psyllids with and without Las bacterium is under investigation using PLFA and cloning/sequencing of the 16S rDNAs. Monitoring was provided by emails and site visits.

4. Accomplishments

Last Modified: 08/22/2017
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