Lethal bronzing spreads to new palm hosts, counties

Palm tree pictured with lethal bronzing disease

A new study adds four additional species to the growing list of palm trees susceptible to lethal bronzing disease, for a revised total of 16. Researchers with UF’s Emerging Pathogens Institute and the Institute for Food and Agricultural Sciences sampled hundreds of palms in Florida to identify the new hosts, all of which are common ornamentals found throughout central and southern portions of the state. They also found the disease has spread to eight new counties.

Florida’s robust nursery and landscaping industry faces new threats due to the emergence and spread of lethal bronzing disease. Caused by a parasitic bacteria, and spread by common sap-sucking insects, the disease kills a growing list of susceptible palm trees.

UF researchers Brian Bahder and Erica Goss teamed up for a new study published in Plant Disease that identifies four additional new palm tree species susceptible to lethal bronzing disease, for a total of 16. They also report that lethal bronzing has spread to include eight new Florida counties.

“Now that the disease has been in Florida for 13 years, it’s had time to spread from where it was introduced,” says Bahder, an assistant professor of insect vector ecology, who led the study. “It jumped into new host plants as it moved into different parts of Florida.”

Goss, an associate professor of plant pathology, and Bahder received funding from the Emerging Pathogens Institute to support this research; their home departments are within UF’s Institute of Food and Agricultural Sciences.

Deadly bronzing

Fronds of palms infected with lethal bronzing disease display a brilliant reddish-bronze hue as they die. It differs from the pigmentation changes associated with natural frond deterioration which skews to gray and brown.

“Lethal bronzing is akin to the tree version of deadly malaria,” Bahder says. “It is spread between hosts by leafhoppers, which are kind of like mosquitoes for plants.”

The palm cixiid, Haplaxius crudus, is thought to be the single vector capable of carrying the bacterial parasite that causes lethal bronzing, phytoplasma 16SrIV-D.

“Planthoppers have a needle-like mouthpart, which they use to feed on the sap of the palm,” Bahder says. “They acquire the phytoplasma when feeding. When they move to a healthy palm to feed, they inject their saliva, and it infects the new palm.”

The reddish-bronze frond, right, displays coloring indicating infection with lethal bronzing while the grayish frond, left, is an example of normal leaf deterioration, also known as senescence.

Phytoplasma 16SrIV-D infects a susceptible plant’s phloem tissues. Antibiotic treatments are ineffective because phytoplasmas lack the cell wall that most of these therapies target.

A clearer picture

To survey the geographic and host spread of lethal bronzing a decade after it was first detected in Florida, the research team systematically collected 189 samples from 11 palm species at the Fort Lauderdale Research and Education Center where the disease is known to be actively spreading. They also analyzed 302 samples, in the vector entomology lab Bahder oversees at IFAS’s Fort Lauderdale Research and Education Center. The samples were submitted by nursery and landscaping personnel spanning 18 counties statewide. (If you have a palm tree you wish to have tested, click here.)

“My predecessor started a database on lethal bronzing, and at one point I realized I was sitting on ten year’s worth of data about where this occurs and how it’s spread both geographically and into new hosts,” Bahder says. “At the same time, when I give extension talks on this disease, homeowners and landscapers always want to know where the disease occurs and what palms are prevalent. I wanted to bring all the data together so we could give a clear picture of the current distribution in terms of counties, published records, my data, and the diagnostic clinic data.”

While the paper is a scientific publication, Bahder views it as much more valuable to the public than some of his other works. 

The research team ultimately identified lethal bronzing disease via molecular analyses in four new palm hosts: the Pindo palm (Butia capitata), Carpentaria palm (Carpentaria acuminata), Coconut palm (Cocos nucifera), and Chinese fan palm (Livistona chinensis). All are ornamental palm trees commonly found in central and southern Florida. They also documented geographic spread of the disease to eight new counties: Collier, Hernando, Jefferson, Martin, Miami-Dade, Monroe, Seminole, and St Johns. 

“In a way, it’s not surprising that it’s infected new host palm species as it spread south,” Bahder says. “Southern Florida has a wider diversity of palm species compared to central and northern Florida landscapes.”

The greater diversity of palm trees in this area may equate to a greater reservoir of potential hosts.

While no one has tallied the actual economic losses from lethal bronzing, the research team estimates it could be between $10 and $100 million due to devastated crops.

Lethal bronzing history 101

In 2006, lethal bronzing was first found in Hillsborough County by Bahder’s predecessor. Nigel Harrison discovered it in Canary Island date palms, edible date palms, wild date palms, and queen palms. Before this discovery, it was known to occur only in Canary Island date palms in Texas. Then, in 2008 it was found to have spread to cabbage palms — also known as sabal palms — in both Hillsborough and Manatee counties. The sabal palm is native to Florida, whereas many other susceptible palms have been introduced as ornamentals. In 2011, it jumped to pygmy date palms in Hillsborough County, and in 2018 the most recent new host (prior to Bahder and Goss’s study) was identified as Bismarck palms in Manatee County.

Other than Florida and Texas, the only other state in the U.S. to harbor this disease is Louisiana, where it has been found in Chinese windmill palms. Outside of the U.S., lethal bronzing disease is only known to occur in Mexico.

A similar story played out a half-century ago when Florida’s coconut palms were strongly affected by a similar disease, lethal yellowing, which is also caused by a parasitic phytoplasma. The Jamaican tall variety of coconut palm was heavily affected, though other varieties remain in Florida. Bahder is careful to point out that this does not mean natural resistance to lethal yellowing emerged. Rather, he suspects the proportion of insects that can carry the causative phytoplasma pathogen is extremely low compared to all the insects that are likely able to carry it. The same may be true for the vector of lethal bronzing.

“While infected trees have 100 percent mortality rate, it may be that only 1 to 5 percent of the vector insects capable of carrying the phytoplasma actually do,” Bahder says.

New directions

More than a decade ago when scientists were first identifying suspected victims of lethal bronzing disease, they lacked the fine-grained molecular techniques that are now available. Earlier studies relied on scanning electron microscopy to detect the presence or absence of phytoplasma, without the ability to readily distinguish between different types. As a result, researchers question whether some of the samples originally diagnosed as lethal yellowing may in fact have been early cases of lethal bronzing.

Bahder has since created a rapid systematic molecular diagnostic test that can differentiate between both lethal yellowing and lethal bronzing.  He and Goss advocate for standardizing tests for lethal yellowing and bronzing to ensure the two diseases are appropriately diagnosed.

Researchers anticipate the disease will continue to spread.

“Two of the most susceptible palm species are being planted up and down the turnpike and I-75. The transportation department is creating a smorgasbord for the vector to move,” Bahder says. “This species is very mobile anyways. Given the amount of traffic moving on these major roadways, it is bound to continue moving into new areas.”

One of those new areas, according to coauthor Goss, could even be your own property.

“Many people, including myself, have plant blindness,” Goss says “Trees may be dying around us, but we don’t notice until they are in our own backyard. This disease is now killing palms not only on roadways but also in Gainesville yards.”

Bahder’s future research looks even further south than Key West: he will next survey for new strains of phytoplasmas, and leafhoppers capable of carrying them, in the Caribbean Islands.

“As part of our collaboration, we are generating genome sequence data for the two phytoplasmas and any other phytoplasmas that Brian finds during his surveys in the Caribbean basin,” Goss says. “The more data we can generate about the biology of these organisms, the better chance we have against them.”

By: DeLene Beeland