Scientists Identify Bacterial Pathogen of Citrus

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 John Hartung. He and colleagues studied plant specimens dating back nearly 100 years that are preserved in a collection, called an herbarium, housed at the ARS Henry A. Wallace Beltsville Agricultural Research Center at Beltsville, Md. Historic specimens are valuable for studying the genetics of plants and their pathogens.

The findings were described in a recent issue of the Proceedings of the National Academy of Sciences.

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 crop.

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.