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Article in the St. Petersburg Times: Saturday, July 12, 2008
Mysterious Disease is Killing State Trees

Florida's state tree is dying, and there's no cure.

A disease that's killing sabal palms was reported in Hillsborough County, but University of Florida researchers who used DNA testing to confirm the deadly phytoplasma don't know how it is spread or how to stop it.

"It's a new disease, and there are no easy answers," said Hillsborough County forest extension agent Rob Northrop.

What they do know: The tall, slender trees that are the source of "swamp cabbage" - the edible hearts of palm -are turning brown and dying.

The disease has been confirmed in sabal palms along Brandon Parkway, in Ruskin's E.G. Simmons Park and along U.S. Highway 41 south of the Alafia River, Northrop said. It was found in Manatee County two weeks ago.

It's hard to determine how widespread the disease is because there are no field tests, and it can't be diagnosed by simply looking at the trees, Northrop said. Confirming the disease requires expensive DNA testing.

The disease is probably caused by the same phytoplasma - a bacterium without a cell wall - that's killing four other palm species: the Canary Island date palm, the edible date palm, the wild date palm and the queen palm, UF researchers said.

Scientists call it the Texas Phoenix palm decline because it was first found in Texas in the 1970s. It was confirmed in Florida about two years ago and has since spread to five west-central Florida counties.

Along Brandon Parkway, two types of palms - the Canary Island date palm and the sabal palm - have the disease, Northrop said. Hillsborough County spent $2.5-million landscaping the 3-mile road in 2003, and those palms are only about 100 yards from newly planted ones on Lumsden Road.

That's too close, if researchers' theories are right. They suspect the phytoplasma is carried by plant-hopping insects that feed on the trees' nutrient-carrying tissue.

Because each tree can cost thousands of dollars, Northrop suggests that people stop planting palms until more is known about the disease.

For now, homeowners who find withering palms in their yard should not immediately remove the tree, Northrop said. Browning leaves and fallen fruit could be caused by other problems, such as nutrient deficiency or over-trimming. Instead, disinfect pruning shears between trimming each palm and consider hiring an arborist to inject an antibiotic.

The antibiotic, called TreeSaver, is inexpensive, but Northrop suggests hiring an arborist to inject it correctly. That could cost a couple hundred dollars and should be done three to four times a year.

The antibiotic isn't practical on a large scale, said Monica Elliott, a UF plant pathology professor based in Fort Lauderdale, so there's no real solution yet for stopping the disease. Meanwhile, palms that grow in the wild are also being affected, Northrop said. Sabal palms make up about 3 percent of the bay area's forest, he said.

"The loss of these palms could change the interaction in the forest, the soil, the water, the animals, not to mention the landscape value of the palms in cities like Tampa," he said.

The next step for researchers is to sequence the phytoplasma's DNA to make sure it's the same as that of the Texas Phoenix palm decline. Then they hope to determine which insect is carrying it and how to prevent spreading.

That will cost a lot of money, Elliott said, and the problem comes at a bad time. The budget of UF's Institute of Food and Agricultural Sciences was recently slashed as the university and the Legislature faced their own spending cuts.

Researchers have been relying on grants, many of them federal, but that doesn't bode well for palm tree research. Because most palms are found in Florida, it's hard to get substantial federal money, Elliott said.

"Our biggest enemy is that it's a local problem," she said.

Fast facts

What's killing the palms?

Phytoplasma - a bacterium without a cell wall. It's probably the Texas Phoenix palm decline, known to scientists as 16SrIV subgroup D.

How it spreads: Scientists aren't sure, but they suspect plant-hopping bugs that suck up nutrients containing the phytoplasma.

Symptoms: They include dead lower fronds, the death of the spear leaf - the newest frond in the canopy - and falling fruit. Sabal palm fronds turn bronze or reddish-brown, then gray-brown.

Spotting an infected tree: The symptoms are a good clue, but nutrient deficiencies or over-trimming also might be to blame. DNA testing gives the only sure answer.

What to do with an infected tree: Remove it immediately, although the extension service suggests not simply pulling out a tree at the first sign of brown fronds.

Fusarium Decline of Queen Palms and Mexican Palms in Florida

Monica L. Elliott, Ph.D. University of Florida - IFAS, Fort Lauderdale Research and Education Center

- History
The sporadic death of the queen palms in one Palm Beach County was first brought to our attention in 2003. Conversations with landscape maintenance staff throughout the southern half of the state, where most queen palms are grown, indicated the problem was observed in other counties at approximately the same time. This meant the 2004 and 2005 hurricane seasons could not be blamed for the spread of the problem. While initially less than 1% of the queen palms died in a single landscape, there are now sites where upwards of 5-10% of the queen palms have died. Untill early 2007, no other palm species had been affected, even in landscapes dominated by a mixture of palm species. Most of the queen palms observed had been in the landscape for five or more years - I.e., it was affecting until early 2007, and it was a container nursery and not a field nursery.

- Host Expansion
In late April/early May 2007, I obtained samples of diseased leaves from Washingtonia robusta from three locations distant to each other - Lee County, Orange County and Pinellas County. Two samples were from palms established in the landscape and one sample was from a field nursery. One of the landscape palms was on a property site where the disease had been observed previously on queen palms. The field nursery had never observed the disease prior to this time, and the disease occurred simultaneously in a queen palm block and a Mexican fan palm block (situated side by side). These palms had been in the field for approximately two years.

- Probable Pathogen (or Patience is a virtue)
I have consistently isolated Fusarium oxysporum from diseased queen palms (mature with trunks) in the landscape since early 2004. However, it was not uncommon for the other "potential" pathogens to be isolated at the same time from the same small piece of tissue, including other Fusarium species. On the other hand, Fusarium oxysporum has been the only "potential" pathogen isolated from the petiole tissue of diseased queen palms growing in a container nursery and, mosy recently, from diseased queen and Mexican fan palms growing in a field nursery (juveniles without trunks).

To confirm which fungus isolated from diseased plant is the pathogen, plant pathologists must complete a series of steps called Koch's postulates. The first step is consistent isolation of a pathogen from symptomatic host tissue. In the second step, the pathogen is grown in pure culture and its characteristics documented. The third step is inoculation of a healthy plant with the pure culture of the pathogen, using the same plant species from which the pathogen was originally isolated. This inoculated plant must then develop symptoms similar to those observed initially. The fourth step is recovery of the same fungus used for inoculation purposes.

Sometimes completing Koch's postulates is a simple task: sometimes it isn't! It has been extremely difficult to complete Koch's postulates with the Fusarium oxysporum and queen palms. We have set-up a series of experiments using queen palms (seedlings and juveniles) and thus far, If the palms die, they are dying from fusarium oxysporum and not from other "potential" pathogens. On the other hand, when Washightonia robusta seedlings were inoculated the first time (using the exact isolates as used for the queen palm seedlings), all inoculated plants died within a month.

Preliminary DNA analysis indicates that the Fusarium oxysporum isolates obtained throughout the state and from both palm hosts are the same. Futhermore, this Fusarium oxysporum appears to be new. It does not appear to match any other known pathogenic Fusarium oxysporum strains, including the strains that cause Fusarium wilt of Canary Island date palm, edible date palm and oil palm. Further DNA analysis is needed to confirm the preliminary results.

- Pathogen x Host(s)
Fusarium oxysporum is not a single fungus, but is composed of subgroups called formae speciales. In general, these subgroups are determined based on the host species affected by the fungus because these subgroups cannot be seperated based on the typical characteristics used to separate fungi. Examples: The primary host of fusarium oxysporum f. sp. canariensis is Phoenix canariensis (Canary Island date palm). The only host of Fusarium oxysporum f. sp. albedias is Phoenix dactylifera (date palm), and this fungus is only found in Morocco and Algeria. The primary host of Fusarium oxysporum f. sp.asparagi is Asparagus officinalis (asparagus). Thus it is quite surprising, but not impossible, to have isolated the same fusarium oxysporum from two different palm species. Whether other palm species will be affected is simply not known.

However, this is an alarming development relevant to management strategies and potential spread of the pathogen. If queen palms had been the only host, then avoiding the planting of queen palms into an affected area would have been a useful long-term strategy. While we have observed the continuing northward movement of queen palms (they really are not very cold hardy) with some dismay, Washingtonia robusta are relatively cold hardy and are found throughout the state. This means the disease can spread throughout the state. In other words, it is no longer just south a Florida problem.

- Symptoms
The problem has always appeared to be a disease. No insects are observed in association with symptomatic palms, nor are nutritional deficiencies implicated in the problem.

Symptoms Begin on lowest (oldest) leaves in canopy.(Figure 1)

Progression of symptoms up through the canopy.
Oldest leaves are on the right and youngest leaves are on the left.(Figure 2)

All leaves in the canopy have died.(Figure 3)

Diseased (left) vs. healthy (right) queen palms.(Figure 4)

- Queen Palms
Symtoms on queen palms (a featherleaf palm) are as follows. The lowest (oldest) 2-3 leaves turn brown (leaflets and petiole) but do not break or hang down (Figure 1). The next 2-3 youngest leaves in the canopy will turn varing shades of yellow. This pattern continues up through the canopy (Figure 2) untill the entire canopy is brown. It looks as if the canopy has been freeze-dried in place (Figure 3 and 4). The time between initial symptoms and palm death is only a few months. The symptoms are different from early senescence of leaves with potassium deficiency, a common nutrient deficiency of queen palms. With severe potassium deficiency, the leaflets will be brown but the petiole will be green. With the new disease, both the leaflets and the petiole are brown.

Brown stripe on petiole extends into the rachis of the leaf.(Figure 5)

Brown external petiole stripe corresponds with internal discoloration of the petiole.(Figure 6)

Leaflets on one side of the leaf have died (right), but leaflets on opposite side are still living (left).(Figure 7)

Closer examination of the yellowing leaves and the next green leaf in the canopy should reveal what is believed to be the initial target of the pathogen - the leaf petiole at the point where it is bending out of the canopy. There are areas of discoloration (brownish-red color) at this point the seem to spread in both directions on the petiole (a stripe), toward the trunk and toward the leaf tip (Figure 5). The petiole is not soft and rotted, but simply discolored. Cross-sections through the petiole reveal internal discoloration (Figure 6). Cross-sections must be made with a sharp saw and not with a crushing tool such as a pruning shears or loopers, as the crushing motion will discolor tissue also. Leaflet tips, even on lower green leaves, may exhibit drought-like symptoms. I have also noted that many leaves initially exhibit one-sided death symptoms, where the leaflets on one side of the leaf are brown (necrotic) but the leaflets on the other side of the leaf are green, with the brownish-red stripe on the petiole corresponding to the side with the dead leaflets (Figure7).

Progression of symptoms up through the canopy.
Oldest leaves are necrotic, and subsequently younger are
shades of yellow and green(Figure 8)

Note brown stripe on petiole of the leaves.(Figure 9)

- Mexican Fan Palms
Symptoms on Mexican fan palms will also have more brown (necrotic) lower leaves than normal and subsequently younger leaves with leaf blades that are partly green and partly yellow or brown (Figure 8). The petioles of these leaves will have brown to reddish-brown stripe running from the blade back to the trunk (Figure 9). Again, it appears to be a relatively short time (a few months) between initial symptom development and palm death. Cross-sections through the striped petiole reveals an associated internal discoloration. The individual leaf symptoms (brown stripe with uneven leaf blade coloration) will look the same as another disease that can be observed on Mexican fan palms, petiole blight. Petiole blight is not normally a fatal disease, wheras Fusarium decline is fatal.

For both palms, the apical meristem (bud) of the palm is not killed until sometime after the canopy turns completely brown. When cross-sections are made through the crown of a dying queen palm, the bud is still clean and white, but older leaf and inflorescence (flower) bases are discolored and usually rotted. The symptoms and their development suggest the pathogen is producing a toxin.

Pathogen Movement

Based on the locations of the disease (both on queen palms and now Mexican fan palms) throughout the southern half of the state, it would appear that the pathogen is most likely spread to new sites by wind. Once the disease is established in a landscape or neighborhood or nursery, it is possible the pathogen could be spread by pruning tools, in addition to be spread by wind. Therefore, as with fusarium wilt of Canary Island date palms, it may be useful to sterilize pruning tools prior to pruning queen palms and Mexican fan palms, even those that appear healthy. If the client insists on having leaves of a diseased palm trimmed (rather than removing the entire palm immediatly), sterilize the pruning tool prior to pruning the next palm. See the table below for materials to use for sterilizing pruning tools.

Table 1.Suggested materials and soaking times for disinfecting pruning tools. Information developed for managing Fusarium wilt of Canary Island date palm.

Material*	%Solution	Soaking Time
Household Bleach (ex: Chlorox)	25% (1 part bleach + 3 parts water)	5 minutes minimum
Pine oil cleaner (ex: Pine Sol)	25% (1 part cleaner + 3 parts water)	5 minutes minimum
Rubbing alcohol (70% isopropyl)	50% (1 part alcohol + 1 part water)	5 minutes minimum
Denatured ethanol (95%)	50% (1 part alcohol + 1 part water)	5 minutes minimum

*The above materials were shown to be effective in eliminating the fungus from the wood dust or palm sap trapped on pruning tools (Simone, 1998). It is suggested that the solution be replaced after ten trees or every two hours. Rinse the tool with fresh water after disinfecting. Other potential disinfectants would be trisodium phosphate or quarternary ammonium salts. The latter is recommended as a 5% solution for 5 minutes (Smith et al., 2003).

Management Strategies

Palms with Fusarium decline should be removed immediately and the canopy destroyed. As long as the disease has not moved down into the trunk tissue, the trunk can be recycled as mulch. Removing the diseased palm removes a potential inoculum source of the disease. However, caution should be used regarding Mexican fan palms. Without a laboratory test, it is not possible to know if a Mexican fan palm has Fusarium decline or petiole blight. If it has petiole blight, the palm does not need to be removed, as it is not likely to die. Until more is known about the disease, it would not be recommended to plant a queen palm or Mexican fan palm back into the site where the diseased palm was located.

No fungicides are recommended. Curative applications would not be feasible since the palms decline so quickly after the disease symptoms are observed. Also, fungicides have not been shown to be effective in controlling similar diseases such as Fusarium wilt of Canary Island date palms.

Summary

The quick-killing disease of queen palms (Syagrus romanozoffiana) has spread to another host, the Mexican fan palm (Washingtonia robusta).
The disease has been observed in landscapes, a container nursery and a field nursery.
The disease has been documented thus far only in the southern half of the state
(Orlando to Tampa to Cocoa Beach and all areas south – inland and coastal). However, it is quite likely the disease will spread further north over time.
Infected palms die quickly, often within a few months of the initial symptoms.
There is no cure once a palm is infected, and no preventive fungicide treatments are recommended.
The disease is probably spread by wind into new sites. Once established, it could also be spread by pruning tools. Pruning tools should be sterilized after each palm is trimmed.
Palms, especially queen palms and Mexican fan palms, should not be replanted into a site where a palm with this disease was removed.
The disease is tentatively called Fusarium decline of palms. The fungus Fusarium oxysporum appears to be the pathogen.

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