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Department of Ornamental Horticulture Fact Sheet
Florida Cooperative Extension Service Institute of Food and Agricultural Sciences
University of Florida, Gainesville John T. Woeste, Dean for Extension
LETHAL YELLOWING OF PALM TREES IN FLORIDA
Lethal yellowing (LY), a relatively new disease to Florida, has been recognized by other names in various Caribbean islands for close to 100 years. Jamaica's "west end bud rot," which appeared in the 1890's was a typical example. A small percentage of Jamaica's coconut palms was initially infected. The disease was not considered a serious problem until the 1950's when it began to spread rapidly in Jamaica and the Florida Keys. Because of Jamaica's economic dependence on the coconut, immediate steps had to be taken. In Florida, the Division of Plant Industry and the University of Florida cooperated in efforts to attempt to eradicate the disease.
As with any new disease, early research efforts focused on the cause. Areas investigated included fertilization and nutrition, fungi, bacteria, insects (weevils, borers, etc.) and nematodes. Initially a virus was thought to cause LY as no other disease causing agent could be discovered. Applications of fertilizers, fungicides, bactericides, and insecticides failed to control the disease. In Key West, 75% of the coconut palms were destroyed. Research from 1955-1965 demonstrated that LY was not a disease whose mysteries were going to be unraveled easily.
Spread of Lethal Yellowing
Little attention was given to LY in Florida during the 1960's as the disease gradually died out in Key West. In 1969 LY appeared in Key Largo and in late 1971 several trees in Coral Gables were diagnosed as having LY. Within months the disease had killed a thousand trees in Miami. In 1972, LY was reported in Broward County, and in Collier and Martin Counties during 1974.
Lethal yellowing is found in most of the Florida areas where coconuts can be grown. There is a possibility for LY to spread outside the range of coconut palms. A disease that affected thousands of date palms (Phoenix canariensis and P. dactylifera) in southern Texas in 1979 is now thought to be the same as LY in Florida. Evidence to support this includes identical symptoms in Texas and Florida date palms, the presence of mycoplasma-like organisms, and the presence in Texas of a species of insect which has been shown to be a carrier (vector) in Florida.
Symptoms in Coconut Palms
Lethal yellowing may easily be confused with other yellowing and defoliating maladies of coconut palms, including fungal bud rot, nutrient deficiencies and insect, nematode or lightning damage. Observations over the years, however, have led to the recognition of four symptom stages for LY in coconut palms.
The first symptom is premature dropping of most or all coconuts, regardless of size. This is termed "shelling" and most of the fallen nuts will have a brown or black water-soaked area immediately under the calyx.
The second stage, usually definitive for LY, is the blackening of new inflorescences (flower stalks). This may be observed as they break through the spathe (the structure which enclosed the inflorescence) and is quite distinctive because healthy inflorescences are golden or creamy yellow. Most male flowers will be dead on the blackened inflorescences and no fruits will set.
In the third symptom stage, from which the disease gets is name, the fronds turn yellow, usually beginning with the oldest fronds and advancing upward through the crown. In some cases, one single frond in the center will turn yellow first giving a characteristic "flag" appearance. Fronds that have yellowed will die, turn brown and hang down. Such fronds fall readily, or are easily pulled off.
Death of the bud occurs about halfway through the yellowing sequence. The newly emerged spear leaf will collapse and may be seen hanging down within the crown. Finally the top of the tree falls away leaving a bare trunk or "telephone pole". Infected trees usually die within 3-6 months after appearance of the first symptoms.
Recognition of symptoms of the LY-like disease in other palms is more difficult. The first two symptom stages are the same as for the coconut palm-the premature dropping of fruit (shelling), and the blackening or necrosis of the new inflorescence.
The third symptom, in many cases the first obvious sign of a disorder, is discoloration of the fronds. At this stage of the infection the symptoms differ for individual species.
In general, frond discoloration due to LY falls into two distinct categories: those in which the fronds turn a golden-yellow before dying, and those in which the fronds turn a greyish-brown. Palms in the first symptom category, in addition to coconut palms, are talipot palms, two species of pritchardia, arikury, windmill, princess, and spindle palms.
In these palms, yellowing of the lower fronds is typical, but often one frond will turn before any others, giving a "flagging" appearance. The fronds remain yellow for various lengths of time before turning brown and dying. The fronds tend to break at the leaf base junction and hang down like a collapsed umbrella. Fronds may cling to the tree instead of falling to the ground.
The yellowing usually advances from the older to the younger fronds with the spear leaf being the last to turn yellow. Once the spear leaf shows symptoms, the crown dies, the top of the tree falls away, and only a topless trunk remains.
The remaining susceptible palms fall into the second symptom category, browning or drying out of fronds. The Christmas palm, or adonidia (Veitchia merrilli) is perhaps the best example of this group. The first two symptom stages in these palms are also similar to those in LY-infected coconut palms. Frond discoloration is not as dramatic or easily detected in the early stages as in coconut or pritchardia palms.
First evidence of infection is a brownish "water mark" along the margin of the pinnae or leaflets. Browning gradually extends to the entire frond giving it a dried out appearance. Older leaves tend to break easily at the junction of the leaf-base and midrib and younger fronds tend to break within the lower region of the pinnae. Unopened inflorescences may have a distorted or twisted appearance. Death of the bud follows and the entire top falls away leaving a bare trunk.
The borassus, cabada, cluster, fish-tail, and the three species of Phoenix palms all show "browning" symptoms similar to those described for the Christmas palm.
Lethal Yellowing Research
The University of Florida Agricultural Research and Education Center in Ft. Lauderdale has beeen conducting LY research since 1972. In 1976 the effort increased as the impact of the disease on south Florida was recognized. This research and studies in Jamaica and Africa have brought a better understanding of the disease. The following points highlight these results:
A. Causal Agent of LY
In 1968, organisms called mycoplasmas were found to cause certain plant diseases. Many diseases previously thought to be caused by viruses have since been found to be associated with mycoplasmas or mycoplasma-like-organisms (MLO's). Evidence suggesting that LY was also caused by these single-celled organisms was reported in 1972. The presence of MLO's within the food-conducting tissues (phloem) of palm trees affected by LY and the response of palm trees to anitibiotic injections (oxytetracycline) were the two pieces of evidence which indicate that MLO's are responsible for LY.
B. Insect Vector
A planthopper identified as Myndus crudus has been shown to transmit the causal agent of LY in replicated tests at the Agricultural Research and Education Center. Myndus crudus was collected from the leaves of palms where LY was prevelant and then transferred to caged, healthy palms. In all cages where the insects were introduced, the palms developed LY. The presence of the MLO was subsequently verified with the electron microscope.
C. Vector Control
Insecticides slow the spread of LY, but they were not found sufficiently effective to warrant use in the field. Also, potential health and environmental effects of many insecticides in an urban area preclude broad use on tall coconut palms. Biological methods of controlling the vector are being investigated.
D. Culture of Mycoplasmas
Because plant mycoplasmas require precise conditions and a living host for reproduction, new methods are being explored to allow rapid multiplication of MLO's for research use. Related mycoplasmas have been cultured using a new technique involving growth in an alternate host. Because of the size of palm trees, the number of years required to grow the trees, and the long incubation period of the disease (6 months to 2 years), a model system has been developed using Aster Yellows, a disease similar to LY that affects herbaceous plants. The mycoplasma that causes Aster Yellows can be isolated, introduced into the known vector, and re-transmitted to healthy plants. Much of the work done with Aster Yellows can be applied to LY research to speed up our understanding of the diseases associated with MLO's.
E. Spread of the Disease
Lethal yellowing has eliminated nearly all of the 'Jamaica Tall' coconut palms in Dade County, but some plantings of 'Jamaica Tall' had escaped the disease by 1981. Broward County has also lost most of its original coconut palms. Palm Beach County, largely because of the success of antibiotic injection programs, had lost only 35-40% of their coconut palms by 1981. Replanting of resistant coconut varieties and the survival of some 'Jamaica Tall' palms have all but restored the tropical atmosphere of the Florida Keys (Monroe County). The west coast of Florida still has most of its 'Jamaica Tall' coconut palms but LY is a continuing threat to this area.
A lethal yellowing-like disease in Texas is killing thousands of Canary Island and some true date palms. This points out the tremendous need for research on LY and raises the possibility that the disease may spread to the commercial date farms in California and elsewhere.
F. Other Palms Affected
Lethal yellowing also affects many other palm trees. Since 1972, 27 species of palms have been added to the LY susceptibility list. Many of these have commercial and ornamental value. Some species such as the Christmas palm (Veitchia merrilli), Fiji fan palm (Pritchardia pacifica), and Canary Island Date palm (Phoenix canariensis) are highly susceptible and it is recommended they not be planted until resistant selections are found. Other palms such as the Chinese fan palm (Livistona chinensis) and Cabada palm (Chrysalidocarpus cabadae) appear to be highly resistant and can still be used in the Florida landscape.
G. Antibiotic Control
Treating palms with oxytetracycline is effective in well-managed injection programs ( see Circular S-228, "How to Treat Your Palm With Antibiotic"). The University of Florida recommends injecting only those coconut palm trees considered valuable to the landscape, and underplanting with resistant varieties for the future.
H. Palms Resistant To Lethal Yellowing
The long-range solution to LY is to develop or introduce resistant palms. To determine what makes a palm resistant a much better understanding of the disease is necessary. Hybrid coconut varieties such as the 'Maypan' are being planted in south Florida and their characteristics observed. Visual surveys of extensive existing palm collections in south Florida indicate that a number of species have potential as new introductions. Work in this area is being conducted both in the field and in a specially designed test garden at the AREC, Fort Lauderdale.
What Can You Do?
To restore the tropical beauty lost to LY, a renewed program of replanting with resistant palm varieties must be undertaken. In the past, the only available coconut palm resistant to LY was the 'Malayan Dwarf'. The new hybrid 'Maypan' appears to require less care and grows more robustly than the commonly available 'Malayan Dwarf'. An increasing number of the horticulturally superior Green 'Malayan Dwarf' have become available. As with most ornamental plants, provide the best conditions at transplanting and a regular fertilization program for optimum growth.
Research is now underway to select new varieties of coconut palms and other palm species resistant to LY. Your county Extension office can suggest suitable palms already available from nurserymen.
*Henry Donselman is Assistant Professor and Extension Specialist, Lethal Yellowing; Department of Ornamental Horticulture, Institute of Food and Agricultural Sciences, University of Florida.The author wishes to express his appreciation to the members of the Lethal Yellowing team: H.G.Basham, Assistant Professor, Physical Biochemist, F.W.Howard, Assistant Professor, Entomologist, R.E.McCoy, Associate Professor, Plant Pathologist; and J.H.Tsai, Associate Professor, Entomologist, for their assistance in preparing this publication.
This publication as promulgated at a cost of $984.00, or 8.2 cents per copy, to inform interested Florida residents about lethal yellowing of palm trees. 10-12M-81