Skip to main content
ARS Home » Research » Publications at this Location » Publication #186024
Title: GENETIC DIVERSITY AND WORLDWIDE PROLIFERATION OF CITRUS BACTERIAL CANKER PATHOGENS IDENTIFIED IN HISTORIC SPECIMENS

Author
item LI, WENBIN - USDA APHIS
item BRLANSKY, RONALD - U. FLORIDA
item Hartung, John
item SONG, QIJIAN - U. MD

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/10/2005
Publication Date: 11/7/2005
Citation: Li, W., Brlansky, R., Hartung, J.S., Song, Q. 2005. Genetic diversity and worldwide proliferation of citrus bacterial canker pathogens identified in historic specimens. Meeting Abstract.

Interpretive Summary:

Technical Abstract: Citrus bacterial canker (CBC) caused by Xanthomonas axonopodis pv. citri (Xac) may have originated in Southeast Asia, based on symptoms present on early herbarium specimens. The disease was first introduced into the United States in 1911 and has spread to most citrus producing areas in the world. This serious disease now threatens the world citrus industry, especially the $10 billion Florida industry. Regulatory emergencies triggered by the appearance of new strains make it increasingly important to understand the origins, evolution and phylogenetics of this pathogen. No information was available on the genetic diversity of the pathogen during the earlier expansion of the range of the pathogen. Herbaria are important resources for the study of the origins and dispersal of plant pathogens. USDA-ARS at Beltsville, Maryland maintains 741 herbarium specimens of citrus with symptoms typical of CBC, originally from 36 citrus producing countries in the world. Since DNA in herbarium specimens is often degraded and citrus plant materials contain compounds that inhibit PCR amplification, we have developed an efficient and reproducible protocol for DNA extraction. We then selected a total of 90 specimens representing 20 citrus and related species from the herbarium collection. Based on the full genomic sequence of reference strain Xac306 of the pathogen, we designed a comprehensive set of primers targeted at transposable element insertion sites located near strain-specific genes. Our method, which we call Insertion Event Scanning (IES), should become a powerful tool for the study of bacterial population genetics. To demonstrate the power of the method we have obtained invaluable data which document the substantial genetic diversity of CBC pathogens deposited as herbarium specimens worldwide during the 20th century. The primary, secondary and tertiary proliferation centers of CBC pathogens have been identified in Japan, the Philippines and India, respectively. Most of the CBC emergencies in citrus producing areas in the new world have been traced back to their origins from the proliferation centers in the old world. A map of the likely routes taken by the bacterium during the course of its global expansion has been developed. IES represents a novel and general approach to the study of bacterial diversity and will be immediately applicable to all bacterial taxa as full genome sequence data becomes available. The method will also be useful in the analysis of the origins and management of contemporary outbreaks of any previously characterized pathogen.