Scientists Identify Bacterial Pathogen of Citrus
By Rosalie Marion
December 17, 2007
Forensic plant pathologists have
identified the original pathogen responsible for the first U.S. outbreak of
citrus bacterial canker (CBC), a disease that historically has imperiled the
Florida citrus industry.
The project was led by Agricultural Research Service (ARS) plant pathologist
Hartung. He and colleagues studied plant specimens dating back nearly 100
years that are preserved in a collection, called an herbarium, housed at the
Henry A. Wallace Beltsville Agricultural Research Center at Beltsville, Md.
Historic specimens are valuable for studying the genetics of plants and their
The findings were described in a recent issue of the Proceedings of the National Academy of
ARS is the U.S. Department of Agriculture's (USDA) chief scientific research agency. The
ongoing project is a collaboration between Hartung and plant pathologist Wenbin
Li, with USDA's Animal and Plant Health
Inspection Service, which provided funding.
The scientists selected the 90 oldest specimens from among 741 preserved
leaves, bark or fruit peels that showed symptoms of citrus bacterial canker.
They carefully cut 10 raised lesions, or cankers, from each selection. Such
cankers weaken trees, induce premature fruit drop and reduce the value of the
The researchers also developed a sensitive new technique for extracting and
analyzing DNA fragments from the removed lesions. The team then matched the DNA
fragments with strain- specific, genetic targets taken from a previously
sequenced CBC strain.
Standard bacterial identification methods require intact DNA that has been
removed from live bacteria. The new technique is called IES, for insertion
event scanning. IES is especially useful for identifying bacterial strains that
are present in preserved specimens, in which the bacteria are no longer viable
and their DNA has been degraded.
By finding an exact match between CBC pathogens from both Japan and Florida
preserved in the herbarium specimens, the researchers revealed the source of
the original outbreak of citrus canker in Florida in 1911.
Using the new IES method to solve contemporary problems could shed light on
how bacteria are disseminated around the world, according to the authors.