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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Subtropical Plant Pathology Research » Research » Research Project #415125


Location: Subtropical Plant Pathology Research

2013 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 is related to inhouse project objective: 1. Characterize ecology, biology, epidemiology, molecular genetics, and vector and host (crop and weed) interactions of domestic, exotic, newly emerging, and re-emerging pathogens. 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. Eight different types (designated as A to H) of Las were identified based on their prophage sequence variations. Sequence analysis of these 8 types of sequences revealed the variations in this region may result from the frequent recombination and reassortment. Typing results using primers specific to the different types of sequences indicated that Type A and B were two major groups that located in FP1 and FP2, respectively. Las-infected periwinkle and dodder contained all types of Las populations; while psyllids only contained type A, B, C and E, and very low titer to none of D, F, G, and H. Las-infected citrus plants contained various types of Las populations, and only the type D population was associated with huanglongbing (HLB) symptoms: high titer of D with typical blotchy mottle, extreme low to none of D with vein yellowing or other atypical HLB symptoms. Similar results were obtained from global samples, indicating the dramatic variations of these prophage regions among the isolates both from Florida and China. These intraspecies variations derived from the prophage activities may be important for the bacterial adaptation to their host plants and insects.

4. Accomplishments