This post complements Chapter 7 Diseases Stalk the Cabbage Palm in my book, The Palmetto Book: Histories and Mysteries of the Cabbage Palm, published by the University of Florida Press.
Disclaimer: I am not a plant pathologist, I am a natural historian. These observations are my own, based on my familiarity with cabbage palms and having attended three day-long UF/IFAS summits (2017,2018, & 2019) on Lethal Bronzing.
Lethal Bronzing is a fatal disease that affects many species of palm trees, including our native cabbage palm, Sabal palmetto. It was first diagnosed in Florida in 2006. It was noticed and diagnosed upstream of Myakka River State Park in in Manatee County late May 2020. It was probably present along the Myakka months, or possibly years earlier.
The disease is a phytoplasma believed to be spread by a small planthopper, Haplaxius crudus, the American palm cixiid (which you can pronounce as SIX-ID). The adults feed mainly on palms, while the nymphs feed mainly on grasses.
According to UF/IFAS handout Lethal Bronzing Disease (excerpt reproduced below) there are only two forms of management at this time: Preventive treatment with the antibiotic Oxytetracycline and removal of afflicted palms.
“Management of LBD involves removal of infected palms and preventative injection of antibiotics. Current data suggests that once palms start showing symptoms, the label rate for the antibiotic oxytetracycline-hydrochloride is not sufficient for symptom reversal. Because of this, upon symptom development and/or a positive test result, a palm is considered lost and should be removed immediately to reduce the amount of time this source of phytoplasma exists in the environment. The longer it is left, the higher probability that further spread will occur. Sampling healthy-looking palms around symptomatic palms can help get ahead of the disease because healthy-looking palms can also test positive. Even though no symptoms are present, those palms still need to be removed because there will not be sufficient time for the antibiotic to take effect before symptoms develop. Also, by testing healthy-looking palms, you can identify which palms are not infected and start preventative injections with the antibiotic.”
The annually inundated floodplain of the Myakka is typically a Live Oak/Cabbage Palm hammock, with a theoretical potential of over 500 cabbage palms per acre in pure stands.
along the park drive in Myakka River State Park.
Since the antibiotic treatment involves injections three or four times a year, it is not practicable to implement a preventive strategy based on injecting antibiotics in to wild cabbage palms in the Myakka landscape. While antibiotics may prevent infection, there is no evidence it can cure (save) an infected tree. In fact, treating an infected palm with antibiotic slows the progression of the disease, delaying the inevitable while increasing the time for transmission to other palms.
Once the central spear leaf (the vertical leaf in the center of the canopy) is dead (brown) the afflicted palm is not considered to constitute a risk to other palms because the insect vectors will not be feeding and then relocating to another plant. They’ve probably already gone.
However, the period between the onset of disease and spear leaf death represents a critical period when insect vectors could spread the disease to other palms. Consequently, the sooner a palm can be diagnosed and all green matter removed, the better.
The lower fronds are dead, as is the central upright spear leaf.
Nothing can save this plant.
Virtually all unpruned cabbage palms have a lower tier of light tan-colored dead fronds in their canopy. When the ratio of dead fronds to live fronds increases, that is a signal of ill health. While many things can kill cabbage palms, a discernible and more or less rapid progression of dead leaves moving upwards (particularly if an unusual reddish bronze color is present) could be interpreted as a death sentence for the tree whether or not the cause is Lethal Bronzing.
Testing the palms for the disease involves taking a sample, keeping it cool, mailing it overnight, and paying a $75 fee for testing. Instructions for sampling. Ideally, suspicious palms should be tested for a positive diagnosis before acting, but if that is not practicable, removal of the living green, leafy portions of such dying trees by decapitation or felling may be the best strategy currently available. It is possible that the Cixiids target the younger, more tender fronds, so removing the emerging spear leaf should be the highest priority. To optimize the process, the removed vegetation should be disposed of in a manner least likely to result in any planthoppers relocating. Current recommendations suggest burning or mulching, both of which may be challenging with green palm fronds.
But, as noted, if the spear leaf has turned brown, is no longer a threat because it is of no interest to the Cixiid planthopper vector.
Aside from removing afflicted palms, the other theoretical approach would be to suppress the insect vector or prevent the movement of vectors or infected palms. In terms of wild hammocks along the Myakka, there is currently no known practicable method for suppressing the Cixiid planthopper that spreads Lethal Bronzing. The planthopper can arrive on vehicles, the wind, or on introduced plant material. Since it is not feasible to manage the movement of vehicles or the wind, the only approach with any likelihood of success would be to limit the movement/introduction of living plant material that could either harbor the planthopper or already be infected with Lethal Bronzing. While it is obvious this would include palm species, other species of plants likely function as asymptomatic reservoirs of the disease. Therefore, the most conservative approach would be to limit or eliminate the importation or movement of living plant material, particularly palms and grasses on which the Cixiid nymphs feed into the area of concern.
My Advice
- Get to know and keep track of your cabbage palms. Don’t just drive by. Familiarize yourself with the normal or typical ratio of dead fronds to live fronds.
- If you suspect an increasing ratio of dead to live fronds, make a note, or, better yet take a cell phone photo from vantage point you are likely to return to.
- If fronds continue to die, you will have to decide if you want to take a sample and get a confirmation (it could be some other affliction) or conclude the plant is on an inexorable decline that will result in death anyway.
- If you conclude the palm has Lethal Bronzing, and it is practicable, cut it down or remove all living (green) portions of the afflicted palm. If that is not possible, pay particular attention to the youngest spear leaf.
- Palms that have already died or have a dead spear leaf are no longer a threat. They provide habitat for woodpeckers and other species and energy spent cutting them down would be better spent on dealing with remaining live trees.
- Consider a moratorium on bringing new plants, particularly new palms or grasses, into the area.
- Alert your neighbors regarding this disease which has the potential to drastically change the hammocks of the Myakka as we know them.
- When communicating with Florida legislators or other elected officials, ask them to support increased funding for research related to Lethal Bronzing. The primary locus of this research is the UF/IFAS Fort Lauderdale Research and Education Center.
The cypress are still looking good around here will go on a walk about to check out the palms