A wild date palm that tested positive for lethal bronzing, next to a neighboring palm that is still green. The bronzing starts in the oldest, lowest fronds and moves up the canopy. All photographs courtesy of Ptolemy Inc.

Two experienced palm professionals looked at the same wild date palm. One is an ISA Certified Arborist; the other has worked with palms for more than 30 years. Working separately, both concluded it had lethal bronzing. The lower fronds had bronzed, and it looked like every textbook photo of the disease.

We sent a trunk core to the lab anyway, and the result came back “Not Detected.” That palm is still alive and in normal condition today. We tell that story because we run into versions of it all the time. Lethal bronzing is hard to manage partly because it is fatal and incurable, but mostly because it is so easy to misdiagnose in both directions.

This photo shows a symptomatic palm that tested positive.

What lethal bronzing is

Lethal bronzing (LB) is a fatal disease caused by a phytoplasma, Candidatus Phytoplasma aculeata, subgroup 16SrIV-D. A phytoplasma is a bacterium with no cell wall. It lives in the phloem, the tissue that carries sugars through the palm, and it cannot survive outside a living host or an insect. It does not move through soil, on pruning tools or in the air.

It was first found in Florida around 2006, near Tampa, where it was known as Texas Phoenix palm decline, and it has since spread statewide. The University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS) lists 21 confirmed host species (Bahder and Helmick, PP163). In Southwest Florida, the palms we lose most often are cabbage palm (Sabal palmetto), wild date palm (Phoenix sylvestris), Canary Island date palm (Phoenix canariensis), true date palm (Phoenix dactylifera) and coconut palm (Cocos nucifera). No species has been shown to resist the disease once infected.

This photo shows a symptomatic palm that tested “Not Detected” and later improved following treatment.

Symptoms follow a sequence. The first sign is early fruit drop and blackened flower stalks, which are easy to miss because crews often prune those stalks off. Next, the oldest, lowest fronds turn a reddish bronze, starting at the tips, and the discoloration moves up the canopy. Finally the spear leaf collapses. That means the palm’s single growing point has died, and the palm cannot recover.

LB is spread by a single insect, the American palm cixiid (Haplaxius crudus), a planthopper about 3 mm (about 1/8 inch) long. Adults feed on palm phloem in the canopy and pick up the phytoplasma from infected palms. The nymphs live in turf and feed on the roots of grasses and sedges, never on palms. Because the immature stages live in the grass, turf management ends up being a large part of control.

Why it is so hard to manage

The first problem is time. Symptoms typically take three to five months to appear after infection, and documented cases have had incubation periods of about nine months (Bahder et al. 2018). During that stretch, the palm looks healthy while planthoppers feed on it and carry the phytoplasma to nearby palms. Infected palms also seem to draw the insect in. In a yearlong UF/IFAS trapping study, cumulative catches of H. crudus were about 4.5 times higher on infected palms than on healthy ones (Mou et al. 2020).

The second problem is look-alikes. Potassium deficiency, the most common one in Florida, also bronzes the lower fronds, but it starts at the tips and margins and does not cause early fruit drop. Magnesium deficiency, normal aging of old fronds, Fusarium wilt, Ganoderma butt rot and Thielaviopsis trunk rot can each resemble LB at some stage. Lethal yellowing is caused by a closely related phytoplasma (16SrIV-A), is spread by the same insect and looks nearly identical in the field. Only a lab assay separates the two.

Third, the disease travels with plant material. A nursery palm can carry the phytoplasma for months without symptoms, and new sod can bring in planthopper nymphs. When LB turns up on a property that has been well maintained and consistently treated, a recent install is often the most likely source.

Put those together and mistakes are expensive either way. A false call can mean removing a healthy palm or drilling it on a schedule for years. A missed call leaves an infected palm standing for months, feeding the insect that spreads it.

Why we say “Not Detected” instead of “Negative”

When a sample comes back clean, we report it as “Not Detected.” That is the term the National Plant Diagnostic Network uses, and we adopted it on purpose. A lab can only tell you it did not find the organism in the tissue you sent. It cannot tell you the palm is free of it. Early in an infection, phytoplasma levels are low and may not have reached the part of the trunk that was cored. “Negative” sounds like a clean bill of health. “Not Detected” describes exactly what the test showed, and it keeps a declining palm on the list for retesting instead of clearing it. You should address terms used, such as “Not Detected,” in your terms and conditions or other contractual documentation for clients. Be clear that your testing and monitoring are limited in scope, and you cannot be held liable for undetected infestations or infections.

Do not inject a palm you have not confirmed

The most common recommendation in our industry is to inject palms with oxytetracycline (OTC) every three to four months as a preventive. Where LB has not been confirmed, that means treating a problem that may not exist, and the treatment carries costs of its own.

Start with the trunk. Palms are monocots with no vascular cambium, so they cannot produce wound wood.

This palm shows successive injection sites.

Injection wounds on hardwoods close over time; on palms they do not (Fishel et al. 2022). A palm injected four times a year for 10 years carries roughly 40 holes.

OTC also does not cure LB. It suppresses the phytoplasma but does not eliminate an established infection. In a UF/IFAS trial on cabbage palms, treated palms still failed and new infections still appeared after a full year of quarterly OTC (Soto et al. 2020).

Injection also can hide the disease. Symptoms are what tell us a palm needs testing, and prior OTC can hold those symptoms back. We have seen palms that were injected with OTC before installation go more than a year after install before their first symptoms showed. During that time the palm looked healthy, and an infected palm that looks healthy can still feed planthoppers and pass the phytoplasma to its neighbors.

When LB is confirmed on a property, injecting every palm is the wrong response. The confirmed palm is the source and should come out. What the property needs next is vector control and testing to find any other infected palms, and injection provides neither.

OTC does have a narrow role. We consider it for a palm that is very valuable or very hard to replace, such as one set in a pool deck, in an elevated planter or reachable only by crane or when it sits in an active zone and its trunk core came back “Not Detected.” Even then, it is the owner’s informed decision.

Our company’s confirm-first protocol

No published zone protocol exists for LB. The steps below are our company’s confirm-first approach, which we have built from vector biology, UF/IFAS research and our own field records. We treat it as a working procedure and revise it as the evidence changes, and no step in it guarantees an outcome.

1. 

   Sticky trap in crown.

   Confirm. A zone opens only on a lab-confirmed positive. Sample the trunk; frond clippings are not reliable. Drill through the outer pseudobark about 2 feet off the ground and collect about 3 grams of living white tissue (Bahder and Helmick, IN1197). Sterilize the bit between palms with a wire brush, alcohol and a flame, because coring can carry Fusarium from palm to palm even though it cannot carry LB. An inconclusive result gets a second core before anything else happens.

2. Draw the zone. UF/IFAS puts the planthopper’s travel at roughly 300 meters (about 1,000 feet). Draw the zone from the positive palm out to that distance and extend it to the nearest hard edge, such as a road, property line or water body.
3. Test the healthy-looking palms, too. Core every susceptible palm in the zone, with or without symptoms, since the palms that matter most are the ones still incubating. On palms without symptoms, we take two to three cores from opposite sides of the trunk, and we use digital polymerase chain reaction (PCR) because it is the more sensitive assay when there is little to find.
4. Suppress the insect, starting in the turf. The population builds in the grass. Treat turf and shrubs, clear grassy ditches and wet spots, and pull mulch rings out to the dripline. Knock down adults in the canopy according to the label, and watch bloom timing and aquatic setbacks.
5. Remove the source, in the right order. Take out confirmed positives quickly. If a positive palm still has a green canopy, knock down the insects on it and its neighbors first. Felling a live positive without that step can scatter infective planthoppers onto the healthy palms around it.
6. Trap, then wait. Hang yellow sticky traps in the canopy after suppression. UF/IFAS identifies what the traps catch and can test the insects for the phytoplasma. We close a zone only after about 60 days of traps with no plant-hoppers, which is roughly one warm-season generation.
7. Keep watching for a year. A palm infected during the outbreak can test “Not Detected” early and turn positive months later (Bahder et al. 2018). We keep up quarterly trapping and close symptom checks for about 12 months after a zone closes. Any positive reopens it.

What managing LB has taught us

A few things have stood out in our field work.

Palms that look the same can test differently. On one property we sampled two wild date palms on the same day with nearly identical symptoms. One came back “Not Detected” and the other came back “Positive”.

Date palms are especially hard to read. On true date palms we have seen the spear leaf collapse while green fronds remained, so waiting for the classic bronze progression can cost time.

We also regularly find sections of soft, spongy fine roots on LB palms. We have not seen this described in the literature, so we treat it only as a supporting sign.

On properties with a history of LB, we hold new palms for seven to 10 days until a trunk core comes back “Not Detected”, and we treat new sod at installation.

Cabbage palms deserve more attention than they get. They are the most common host and the least watched, because few budgets can cover protecting thousands of them. We keep every cabbage palm on a monitoring schedule even when we do not treat it.

Protection lowers risk; it does not remove it. So far, none of the palms we keep on an imidacloprid drench program has tested positive. We find that encouraging, and it is also the reason we keep monitoring them. It is important to keep in mind the secondary effect that imidacloprid and similar systemics may have on pollinators.

Conclusion

LB can be managed, but it cannot be cured. Managing it means keeping the vector population down, removing infected palms quickly and testing long enough to catch infections that were already incubating. All of that depends on knowing what you are dealing with, and that starts at the lab.

Jeff Kaulbars is a partner and lead arborist at Ptolemy Inc., a palm and tree health care company serving Southwest Florida and the Treasure Coast. He is an ISA Certified Arborist (FL-10118A) and holds the ISA TRAQ and Florida Chapter ISA PPQ qualifications and the LIAF credential. He is also an approved CAM continuing-education instructor. He has worked in the lawn, landscape and tree care industry for more than 30 years.

References

Bahder, B.W., and E.E. Helmick. Lethal Bronzing Disease (LB). UF/IFAS Extension, PP163. https://edis.ifas.ufl.edu/publication/PP163

Bahder, B.W., and E.E. Helmick. 2018. Sampling palms for lethal yellowing and lethal bronzing phytoplasmas. UF/IFAS Extension, IN1197. https://doi.org/10.32473/edis-in1197-2018

Bahder, B.W., E.E. Helmick, S. Chakrabarti, S. Osorio, N. Soto, T. Chouvenc, and N.A. Harrison. 2018. Disease progression of a lethal decline caused by the 16SrIV-D phytoplasma in Florida palms. Plant Pathology 67:1821-1828. https://doi.org/10.1111/ppa.12882

Bahder, B.W. American palm cixiid, Haplaxius crudus. UF/IFAS Extension, IN704. https://edis.ifas.ufl.edu/publication/IN704

Fishel, F.M., A.G. Dale, and R.W. Klein. 2022. Pesticide injection and drenching. UF/IFAS Extension, PI274. https://doi.org/10.32473/edis-pi274-2022

Mou, D.-F., C.-C. Lee, P.G. Hahn, N. Soto, A.R. Humphries, E.E. Helmick, and B.W. Bahder. 2020. Effects of lethal bronzing disease, palm height, and temperature on abundance and monitoring of Haplaxius crudus. Insects 11(11):748. https://doi.org/10.3390/insects11110748

Soto, N., A.R. Humphries, D.-F. Mou, E.E. Helmick, J.P. Glover, and B.W. Bahder. 2020. Effect of oxytetracycline-hydrochloride on phytoplasma titer and symptom progression of the 16SrIV-D phytoplasma in cabbage palms from Florida. Plant Disease 104:2330-2337. https://doi.org/10.1094/PDIS-01-20-0029-RE