Study Supports Single Introduction of Laurel Wilt Pathogen in the U.S.

Pathogen's low genetic diversity is good news for the laurels

Nearly one-third of all redbay trees, like this one in Florida, have been lost to laurel wilt. Photo courtesy of Mary Keim.

Laurel wilt has devastated plants in the Lauraceae family – redbay, sassafras, pondberry, avocado, and others – since it was first detected in the southeastern U.S. around 2002.

The disease is caused by the pathogen Raffaelea lauricola and carried by the redbay ambrosia beetle – and by humans moving infested wood.

There is no widespread, effective treatment for laurel wilt. Genetics research is focused on learning more about the pathogen’s genetic structure in order to improve detection methods and screening for possible resistance in Lauraceae host species.

“We have developed genetic markers to describe the population of the pathogen in the U.S.,” says USDA Forest Service plant pathologist Tyler Dreaden. “Knowing which genotypes to use contributes to a quicker, more cost-effective resistance screening process.”

Dreaden led a new study to shed light on the genetic structure of the pathogen and its reproductive strategy. The research team included Marc Hughes at the University of Hawaii at Manoa, Randy Ploetz and Jason Smith at the University of Florida, and Adam Black, horticulture director of the Peckerwood Garden Conservation Foundation in Texas. Their findings were published in Forests.

Dreaden sequenced a portion of the pathogen’s genome in 2014 as a PhD student at the University of Florida as part of a project to develop a method to detect the pathogen.

The researchers extracted the fungal pathogen from diseased wood samples and isolated portions of its DNA for their study. They then designed, tested, and documented a combination of gene sequences – known as microsatellite loci – found only in the pathogen. The new approach greatly reduced the time needed to detect the pathogen and diagnose the disease.

Redbay ambrosia beetles bore into a host tree’s sapwood, then the fungal pathogen clogs its circulatory system and restricts water flow. USDA photo by Stephen Ausmus.

Recent research, utilizing Dreaden and colleagues’ previous work, suggests that the pathogen entered the U.S. a single time and has reproduced in the U.S. asexually.

“We wanted to confirm those findings with a robust study that included more markers and more recent isolates,” says Dreaden. The team developed a multiplex marker panel to study different populations and identify distinct genotypes.

They examined isolates from Japan, Myanmar, Taiwan, and five southeastern states. The scientists analyzed 12 microsatellite loci, combined in eight multiplex polymerase chain reactions.

Their analyses revealed that most of the U.S. isolates were from the same genotype, and all had only one mating type. Two mating types are needed for sexual reproduction. They found multiple genotypes and two mating types in the Asian isolates.

“Our findings support the single U.S. introduction hypothesis,” says Dreaden. “It’s possible that the U.S. genotype originated in Taiwan.”

Remember, don’t move firewood. Buy it where you plan to burn it. CC0 photo.

More research is needed to confirm this hypothesis. The fungal pathogen and beetle are also native to China and India, but no isolates from those locations are available for analysis.

The researchers determined that the pathogen’s U.S. population has extremely limited diversity and one mating type. Dreaden says this low genetic diversity is good news.

“Right now, we can screen surviving redbay with a single isolate, but the pathogen has much greater genetic diversity across its native range. It’s important to prevent those other genotypes from entering the country and to slow the spread of laurel wilt by not moving infested wood materials. That will buy some time for the researchers developing tolerance in redbay, sassafras, and other host species,” adds Dreaden.

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For more information, email Tyler Dreaden at

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