US palm trees under threat from wood-decaying fungus

More than 65 species of palm trees in the United States are vulnerable to a wood-decaying fungus that can damage or destroy palms.

The fungus, Ganoderma zonatum, which causes the lethal disease known as Ganoderma butt rot of palms, shows few symptoms before you can detect something is wrong.

Its mysterious nature has stunted research for decades, making early detection of the silent killer impossible – until now.

A University of Florida scientist in Fort Lauderdale has developed the first-of-its-kind DNA-based diagnostic method that confirms the fungus in the palm months before the symptoms appear.

Published in the journal of Plant Disease, the scientist utilised previously compiled sequence data from genetically validated North American Ganoderma species to develop the tool.

The result is a diagnostic protocol that can detect the genetic make-up of the lethal Ganoderma zonatum pathogen.

“We were able to find the unique genetic markers exclusive to Ganoderma zonatum,” said Braham Dhillon, a molecular plant pathologist at the UF/IFAS Fort Lauderdale Research and Education Center.

“This has great implications for developing management methods for the disease from this point forward.

“It saves time and money and opens the doors for additional research on understanding how this pathogen survives and spreads in the landscape.”

If you suspect a palm has Ganoderma, a tree specialist can collect samples as wood-shavings from the trunk and submit them to the palm mycology lab for diagnostic at the research center.

The procedure reduces the time needed for diagnosing Ganoderma butt rot to a week from when samples are received.

This fruiting body known as a conk, or basiodiomata, typically appears at the lower surface of the trunk confirming the presence of the disease.

The detection method can be readily adopted into protocols of plant diagnostic facilities since the technology and equipment are routinely available without the need for additional instrumentation or chemicals.

“Early detection of the fungus, or any disease, is a crucial step towards building and implementing better disease management strategies and mitigating potential risks from palm deaths and destruction of property due to palm tree decay,” said Mr Dhillon.

Palm trees of all varieties grace the lands of homeowners, public parks, business complexes and roadways. The fungus, common in homeowner and public spaces, is a slow-growing pathogen, that occupies the trunk and degrades the vascular water-conducting tissue.

This produces initial symptoms of wilting and dying palm fronds in the lower part of the canopy.

These symptoms are also associated with other diseases like Fusarium wilt and Lethal bronzing, making it difficult to properly diagnose.

The only symptom that differentiates the presence of Ganoderma butt rot in palms from other diseases is the appearance of a fruiting body known as a conk, or basiodiomata, at the lower surface of the trunk. This symptom appears at the latest stage of the disease.

“The conk is the only distinguishing symptom of the disease and indicates a palm tree is not recoverable,” said Mr Dhillon.

“The conk produces millions of spores that can travel by wind contributing to disease spread.”

As the fungus moves up the trunk, it compromises the structural integrity of the palm, said Mr Dhillon.

“In later stages of the disease, the decayed palm trunk is susceptible to breaking and becomes a hazard to properties, pedestrians and vehicular traffic.

“Depending on the girth of the trunk, the decay process can take up several months to a year.”

Until now, the appearance of the conk, or an invasive dissection of the infected trunk and culturing of the fungus, are the only ways to confirm the diagnosis of the lethal disease, explained Mr Dhillon.

For the study, scientists sequenced a variety of samples, including 24 cultures from 15 Ganoderma species collected from a previous study and archived at the Center for Forest Mycology Research Culture Collection and Herbarium, US Department of Agriculture Forest Service.

Healthy and naturally infected sawdust palm samples from eight palm species were also collected.

Infected palms used were categorised for one of two symptoms: wilted palm fronds or presence of a conk.

Other samples included conks, infected tissues, soil and DNA from palm-infected lethal yellowing and lethal bronzing specimens.

Researchers validated the method on DNA isolated from 60 samples.