Monday, Sep 17, 2007
Posted on Sun, Sep. 16, 2007
Serious diseases threaten palms on Florida's west coast
Sometimes the news I bring is not so good. This is one of those times. Two serious palm diseases have recently been diagnosed along the west coast of Florida, including Manatee, Hillsborough and Sarasota counties. One is lethal yellowing and the other is Texas Phoenix decline.
Both diseases are caused by bacteria-like pathogens called "phytoplasmas." The one that causes lethal yellowing is similar to the one that causes Texas decline, but it's genetically distinct.
The two phytoplasmas are "vectored," or carried from one palm to another by small insects called planthoppers. As they sip the sap from an infected palm, they take up the phytoplasma and then pass it along to a healthy palm during the feeding process. The planthopper that causes lethal yellowing is well known and studied; the one that causes the Texas decline has not yet been identified. These diseases are not transmitted by pruning tools or other types of maintenance.
This is not a new disease to Florida, but it's new to this area. It was first observed in southeast Florida in the 1970s, where it changed the landscape forever by destroying the tall type of coconut palm. It's known to affect 36 palm species. Highly susceptible palms include coconut, Adonidia or Christmas palm, Phoenix dactylifera, and Pritchardia species.
For many years, we did not have positive lab results for lethal yellowing north of Lee County. Now, though, it's been diagnosed in Canary Island date palms at two locations here. How did it get up here? One possibility is that the planthopper vector, which is sensitive to cold temperatures, has successfully moved north with the warmer winters. At this time, we don't know the distribution of lethal yellowing in this area.
Texas phoenix decline
University of Florida research pathologists first learned about this disease problem in late 2006. Thus far, they believe it arrived in west Florida through the infected insect vector before the busy hurricane seasons of 2004-2005.
It's been diagnosed on three species of single-trunk date palms: Phoenix canariensis, P. sylvestris, P. dactylifera; and also queen palm. Locations are in a swath from the Sarasota-Bradenton Airport north to Tampa International Airport.
Since the Texas Phoenix decline is a new disease problem, many unknowns remain, including what other palm species may be affected.
Sampling and identification
The only way to positively confirm the presence of either disease is through molecular techniques used on internal trunk corings. At this time, the corings are sent (by pre-arrangement only) to a lab at the University of Florida/Fort Lauderdale Research and Education Center for analysis ($75 fee).
The lab technique determines only if a phytoplasma is present; it does not show if it's lethal yellowing or the Texas Phoenix decline. An additional lab test can determine exactly which phytoplasma is present.
Source for this article: Dr. Monica Elliott, pathologist, University of Florida/Fort Lauderdale Research and Education Center.
Peggy Dessaint is an extension agent in commercial landscape horticulture at the University of Florida-Manatee County Extension Service in Palmetto. Send your gardening questions to: Peggy Dessaint, Manatee County Extension Service, University of Florida/IFAS, 1303 17th St. W., Palmetto, FL 34221.
The symptoms of lethal yellowing vary by palm species and cultivar. However, the symptoms on the Phoenix species are exactly the same as those associated with the newly discovered Texas Phoenix decline.
First, there's the premature drop of most or all fruits and premature death of flower stalks (inflorescences). Please remember that these symptoms won't be seen where palms are frequently pruned to keep them neat and tidy, or it's not the flower and fruit season.
Next, reddish-brown to gray discoloration of the lowest fronds appears, beginning at their tips. This doesn't look like the more common magnesium or potassium deficiencies.
The fronds turn completely brown. There are more dead ones than you'd expect on a healthy palm that's normally shedding lower fronds. This field observation will be impossible if someone continually prunes off dead fronds.
The tall thin spear leaf (the one that rises up from the center of the palm) dies when approximately one-third of the oldest fronds have died. At this point, the palm stops growing. The spear leaf is difficult to see on Phoenix species due to many spiny and tightly bound fronds at the top.
The remaining green fronds in the middle of the canopy gradually turn brown, from the bottom up.
Management in landscapes
Treatment for both lethal yellowing and the Texas Phoenix decline are the same.
Remove the palm if 25 percent or more of the fronds are already dead, or if the spear leaf is already dead.
If only a few of the fronds are dead and the spear leaf is healthy and green, then trunk injection with an antibiotic called oxytetracycline HCL (OTC) may be used. The OTC product moves through the vascular system, keeping the phytoplasma under control. Injections must be done every four months for the life of the palm.
If a palm tests positive and other susceptible and highly desirable palms are in the neighborhood, then consider treating the healthy palms with OTC injections for prevention. How far away from an infected palm should healthy-looking palms be injected? This remains an unknown.
Insecticides to control the planthoppers, either sprayed on the canopy or used as a soil drench, are not recommended due to limited effectiveness.
As you can imagine, this is a new and expensive chapter in palm care for west central Florida. We are in unchartered territory. Landscape managers and property owners have many questions and concerns. Stay tuned as more research results become available.
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Descriptions of pictures Photo credit: Peggy Dessaint
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