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Coconut Notes
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Dr. Timothy Broschat on Lethal Yellowing
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Posted by:Sep 21st 2003, 06:01:29 pm
jrollsReport on Lethal Yellowing and Other Problems on Coconut Palms on Harbour Island, Bahamas

Timothy K. Broschat, Ph.D.
Professor of Tropical Ornamental Horticulture
University of Florida—FLREC
3205 College Avenue
Ft. Lauderdale, FL, USA 33314
Phone 954 577 6330
FAX 954 475 4125
Email: tkbr@ufl.edu

Introduction

At the request of the South Bar Club, I surveyed Harbour Island, Bahamas in its entirety, but with emphasis on South Bar, on 7-8 Sept. 2003 to determine the status of lethal yellowing (LY) and other diseases and disorders of palms on the island. I found that LY exists throughout the island, most prevalent in areas with concentrations of coconut palms. Lethal yellowing has the potential to kill 90% or more of the coconut palms in any infected area. Since coconut palms are an important component of the of the tropical look that tourists have come to expect, the loss of these palms could potentially have serious negative consequences for the tourist industry on Harbour Island.

Lethal Yellowing—What is it?

Lethal yellowing is a disease caused by a phytoplasma, which is essentially a bacterium that lacks a cell wall. It is spread by a tiny (3/16” long) planthopper insect that feeds on the sap in the palm leaves. . The LY phytoplasma can survive only in living plants or in the planthopper. It does not exist in the soil, water, or air and cannot be transmitted from diseased palms to healthy ones by pruning tools or any other method than the planthopper.

Early symptoms of LY in coconut palms include premature nut drop and the blackening of the young flower stalk tips. Fallen green coconuts from an LY-infected palm often have a blackened area at one end of the nut. As the disease progresses, leaflets on older leaves may show brown areas near the rachis or central leaf stalk. The entire leaf then quickly turns yellow, then brown, and typically hangs down. This leaf death continues to affect progressively younger leaves within the canopy and when only a few (2-4) healthy leaves remain on the palm, the spear leaf (youngest unopened leaf) dies and hangs down. The base of this dead spear leaf will be soft and rotten if pulled out and will have a foul odor. At this point, the palm is dead.

Other palm species besides coconuts are also susceptible to LY. The Christmas or Manila palm (Adonidia merrillii) is at least as susceptible as the Jamaican Tall coconuts planted throughout Harbour Island. In this species, old or mid-canopy leaves will snap somewhere along the leaf stalk and will hang down limply along the trunk. Yellowing of the leaves does not occur in this species. The only other susceptible palm commonly planted on Harbour Island is the Chinese fan palm (Livistona chinensis). In this species, older leaves show premature yellow to brown discoloration and die. Over the course of about 4 to 8 weeks, the entire canopy will die from the bottom up.

Lethal yellowing tends to pass through areas that have high concentrations of susceptible palms in “waves” that typically last up to ten years in any given area. During that time, some groupings of susceptible palms may remain relatively untouched by the disease, whereas other groups may experience losses of 90% or more. Tall coconuts seem to be preferred by the planthopper carriers over short palms, and coconut palms less than 5’ in total height rarely succumb to this disease. The fact that some tall coconuts may survive an LY “wave” does not mean that such a palm is resistant to the disease, but simply that by chance alone, the planthoppers either did not feed on, or did not infect that particular palm. The next wave of LY, which may come 20 or 30 years later, may very well kill those palms passed over by the first disease wave.


Management of LY

Although LY is generally considered to be an incurable disease, injection of palms with antibiotics such as tetracycline or oxytetracycline can result in symptom remission in most trees in early stage if the disease has not progressed to the point where the leaves begin to yellow and die. Dosage Rates 3 times higher than recommended have been shown to be curative in some palms.

Antibiotic injection has traditionally been considered to be a stop gap measure used to keep valuable specimen palms alive until they can be replaced by disease resistant palms. Although the Malayan Dwarf and Maypan coconut varieties have long been considered to be highly resistant to LY, recently published results of a 20-year study in Florida and reports from Jamaica, where these two varieties have been extensively planted, showed that losses up to 99% have occurred in plantings of these two varieties. Since there are no other coconut varieties known to be resistant to LY, planting of resistant coconut varieties is not a viable option. However, there are hundreds of other species of palms that are not susceptible to LY that could be planted instead of coconuts. Increasing species diversity has always been a good method for reducing the impact of a disease epidemic. Some similar-sized palms that could be planted instead of coconuts include Alexandra palm (Archontophoenix alexandrae), Bismarck palm (Bismarckia nobilis), sabal palm (Sabal palmetto), royal palm (Roystonea regia), Veitchia species, and triangle palm (Dypsis decaryii).
In situations where only coconut palms are desired, they can be maintained only by regular (every 4 months) injection with antibiotics.

The simplest method for injecting antibiotic solutions into palm trunks is to use Mauget injectors. A small (1/4” diameter X 1” deep) hole is drilled horizontally into the trunk about 1 to 3’ above the ground. It may be useful to mark off 1” on the drill bit to use as a depth gauge. When repeating injections, it is desirable to locate new injection holes either on a different side of the trunk or at a different height (or both) to reduce damage to one particular region of the trunk. The small aluminum tube is pushed into the hole leaving the tapered end upright and facing away from the trunk. The larger part of the two-piece plastic cup is filled (it holds about 15 ml or cc of solution) with antibiotic solution. One liter of antibiotic solution will treat about 65 palms. To make up a liter of solution that will provide 2 g of tetracycline per tree, mix 133 g of 100% tetracycline or oxytetracycline in one liter of water. For 3 g of antibiotic per tree, add 200 g per liter, and for 4 g of antibiotic per tree, add 266 g per liter.

One level tablespoon of tetracycline weighs about 16 g, _ teaspoon weighs about 2 g, 133 g is equivalent to a volume of about160 cc (or ml), which is about 2/3 cup. It is best to mix up as much solution as will be needed to treat all the trees at one site and pour the solution into the individual injectors, rather than trying to make up numerous 15-ml batches. The dosage to be given will depend on tree size and the stage of disease development. A dose of 2 g per tree is suitable for prevention of LY in a small (less than 5’ of trunk) tree that shows no symptoms of LY, 3 g per tree would be suitable for prevention of LY in a symptomless tall tree or for treating an early stage (only flower blackening is observed) infected small palm, and 4 g per tree may be needed to treat an early stage large palm. Palms should only be injected into the gray wood trunk.

To assemble the injector, pour the appropriate solution into the larger plastic cup of the injector and cover with the smaller one, which fits into a groove in the larger one. Place the filled and covered plastic cup on a hard surface and push the small cup lid down firmly until it locks into place all the way down. Stepping on the cup may be necessary to fully compress the two cups. The small receiving hole at the bottom of the loaded cup is then pushed firmly onto the end of the aluminum tube protruding from the trunk. This will puncture the seal at the bottom of the cup and allow the pressurized antibiotic solution to flow into the trunk. The cup should be in an upright position on the tree with the tube at the bottom. Appendix A lists some sources for Mauget injectors and tetracycline antibiotic.

Antibiotic injection will prevent uninfected trees from contracting LY and will often cause symptom remission in early stage infected palms. If antibiotic treatments are discontinued, infected palms may then die. Although treatment with antibiotics at 3 times the recommended dosage is believed to be curative for LY-infected palms, this has never been scientifically tested. The stage of disease development at which antibiotic treatment is no longer effective is a function of treatment dosage and the stage of disease development. Certainly, palms in which the spear leaf is dead cannot be saved with any treatment. If only a couple of older leaves are yellow or dead, recovery is often possible with prompt treatment using a higher dosage of antibiotic.

Injection of coconut palms with antibiotics is not a perfect solution to the LY problem. First, one cannot continue to inject a palm tree indefinitely. Where this has been tried, palms start to die after about 10 to 15 years due to the physical damage done to the palm trunk by the antibiotic injections. Secondly, coconut fruits, or their milk or water, should not be consumed by humans if they have been injected with antibiotics. To prevent possible liability, I recommend removing all coconuts from injected trees before they reach an edible (or drinkable) stage. This should be easier than trying to educate all possible people (members, guests, employees, or trespassers) that they should not consume these coconuts. Unfortunately, palms only slightly smaller than these are susceptible to LY and thus this stage will remain vulnerable. Finally, injection of all coconut palms of a susceptible size located within the maintained landscape areas surrounding each house would not be difficult. However, large numbers of naturalized coconut palms also occur in non-maintained forest areas of South Bar Club where injection of all palms of susceptible size may not be practical. If these sites are not also treated, they will keep the LY disease active in the area for a much longer time than if all susceptible palms are injected.

Besides antibiotic injection, prompt removal of late stage infected palms, or those deemed to be expendable, will help to reduce the spread of the disease. At least the tops of infected palms that have been cut down should be hauled away to a compost pile if possible. Cutting off the leaves will hasten their drying and render them unattractive to leafhoppers. There is no need to burn infected trees, since the planthoppers that spread the disease will not feed on dead palm leaf tissue. In natural areas where cut infected palms cannot easily be hauled away, they can be left on site it their leaves have been cut off to speed up their drying.

The presence of non-maintained naturalized coconut stands in the forest areas of Harbour Island makes practical control of LY more difficult, since the disease will find refuge in these untreated naturalized coconut stands. Eliminating all susceptible-sized coconut palms would help the situation, although younger seedlings that are constantly germinating under these will continue to provide new susceptible palm hosts for the disease. Perhaps a combination of selective cutting of unnecessary naturalized palms and injection of others deemed to be desirable would be the best solution to this problem.

Other Diseases, Disorders, and Problems of Palms on Harbour Island

In addition to LY, several other problems were noted on palms on Harbour Island. Perhaps the most widespread problem is potassium (K) deficiency. Potassium (also called potash) deficiency is very common on palms throughout the world and Harbour Island is no exception. Symptoms appear first on the oldest (lowest) leaves and the number of leaves showing symptoms is related to the severity of the deficiency. Early symptoms in coconut and many other palms include yellow-orange translucent spots or blotches on older leaves that are visible when the leaf is held up to the light and viewed from below. Another common symptom on most palm species is leaflet necrosis (dead, brown tissue) at the tips or along the sides of the leaflets. Both of these symptoms are more severe towards the tips of older leaves and are less severe toward the bases of these leaves.

Potassium deficiency commonly occurs on sand or porous limestone (coral) soils in Florida and throughout the Caribbean region, since nutrients such as K are easily and rapidly leached through these soils with rainfall or irrigation. Because of this, water-soluble K fertilizers are rather ineffective in supplying plants with K. Slow release or controlled release fertilizers are much more effective in providing nutrients to plants growing under these soil and rainfall conditions. Potassium deficiency can be aggravated by using fertilizers that contain more nitrogen (N) than K, and those that contain slow release N, but water soluble K. Such fertilizers are commonly used on turfgrass and as general purpose fertilizers. Applying these fertilizers directly to palms or to turfgrass within 30’ of a palm can induce severe K deficiencies in palms, which can be fatal. For this reason, we now recommend using only fertilizers that contain 100% of their N, K, and magnesium (Mg) in controlled release form. An analysis (the three numbers on the fertilizer bag label) of 8-2-12 with 4% Mg has been shown to work very well on a wide range of plant species from palms to turfgrass to broadleaf trees and shrubs in south Florida. These fertilizers also contain relatively large amounts of water-soluble manganese and iron, the latter being in the highly effective chelated form. A list of companies that blend 8-2-12 “palm special” landscape fertilizers according to our specifications is given in Appendix B.

Research has shown that the most effective method of applying fertilizers to palms (and other plants within the landscape, including turfgrass) is to broadcast the fertilizer at a rate of 1.5 lbs per 100 sq. ft. using a properly calibrated rotary spreader. We do not recommend applying “turf fertilizers” to turfgrass and palm fertilizers only to palms, since the roots of large palms can extend out up to 50’ from the trunk and will take up whatever inappropriate fertilizers are applied to the turfgrass, often with negative or fatal consequences. Thus, a single highly effective fertilizer (8-2-12 +4Mg “palm special” landscape fertilizer) can be used throughout the landscape on all plants. Use of less expensive fertilizers that don’t have these properties may seem to be economically preferable, but since these products are known to do more harm than good to palms, they are no bargain at any price.

Related to the K deficiency problem on palms is the practice of removing the older leaves of palms that are discolored due to K deficiency. Although this may improve the appearance of the palm in the short term, it is detrimental to palm health in the long term. The reason why older leaves of K-deficient palms are discolored is because, under deficiency conditions, the palm is able to remove K from the older leaves and use it to support the growth of new leaves. If you remove these older discolored leaves, which are providing K for new growth, the palm will then remove K from progressively younger leaves that were previously green. These younger leaves will then become discolored and therefore, candidates for removal as well. Research at the University of Florida has shown that regular removal of K-deficient discolored leaves will result in the premature death of such palms. Palm leaves held at angles above the horizontal (9:00 and 3:00 positions on a clock) should NEVER be removed, and preferably only old COMPLETELY DEAD leaves should ever be removed. Potassium deficient leaves are not dead and are serving an important purpose for the palm. It is preferable to properly fertilize the palms to prevent K deficiency in the first place. Then you will have a palm with green leaves right down to the bottom of the canopy and leaves that die naturally will fall down naturally and will not require pruning.

One other problem I frequently encountered on Harbour Island is the practice of deep planting of coconut palm seedlings, as well as other plants. Planting plants even a couple of inches too deep causes root suffocation and the result is usually plants that grow slowly, show chronic nutrient deficiencies, or slowly decline over time. Coconut and other palms should be planted such that the base of the shoot (where the roots are attached) is about 1” below the soil surface. Depending on where on the coconut that the shoot emerges, the coconut itself may be mostly or partly exposed when properly planted. For trees, the top roots should be right at the soil surface and never more than 1” below the surface.

Although LY is a very serious disease of coconuts and a few other important palm species, there are four other diseases of landscape palms in Florida that are as devastating or more so than LY. These are ganoderma butt rot, thielaviopsis, phytophthora bud rot, and furarium wilt of date palms. All but the last disease affect most species of palms and all are fatal, soil-borne diseases that are impossible to cure and difficult to impossible to prevent. If field-grown palms are imported from Florida into the Bahamas, it is quite likely that one or more of these diseases will eventually be imported as well and that could be disastrous for palms in the Bahamas. I highly recommend that some type of restrictions be placed on the importation of field-grown palms from Florida to prevent the entry of these other serious palm diseases into the Bahamas.

Conclusions

Lethal yellowing is fairly common on Harbour Island, especially in areas where larger concentrations of coconut palms are found. If this disease is not confronted quickly, losses could be high and the practice of replanting with more coconut palms will only keep the disease active in the area for a longer time. Since there are no resistant coconut varieties with which you can replant, injection with tetracycline or oxytetracycline every four months along with prompt removal of late stage infected or expendable palms appears to be the best solution. Replanting with other LY-resistant species of palms or even broadleaf trees is highly recommended.

Potassium deficiency is widespread on palms (and some other plants) throughout Harbour Island. Potassium deficiency is normally no more than a cosmetic problem and palms in nature can live with it indefinitely with mild to moderate symptoms. However, if older, discolored leaves are regularly removed, or if improper fertilizers are used near these palms, K deficiency can kill these palms. Potassium deficiency can be prevented or corrected by using a special controlled release 8-2-12 “palm special” landscape fertilizer.

Deep planting of coconut palms and other species of plants is a widespread practice on Harbour Island and it needs to be stopped. Although plants may survive deep planting, their growth rate and nutritional health will suffer as a result.

Harbour Island does not appear to have any of the other four serious palm diseases that are devastating palms in Florida and I recommend that some type of restrictions be placed on the importation of field-grown palms from Florida to prevent these diseases from becoming established in the Bahamas.

Appendix A.

Some sources for Mauget injectors.

J.J. Mauget Co. Professional Tree Care
5435 Peck Rd, 600 Lake Catherine Dr.
Arcadia, CA 91006 Maitland, FL 32751
Phone 626 444 1057 phone 407 647 3335
FAX 626 444 7414 FAX 407 647 6211



Some sources for tetracycline or oxytetracyline (100%)

Fishman Chemical LLC The Fishy Farmacy
215 Ojibway Ave. 11660 E. Camino del Desierto
Tavernier, FL 33070 Tucson, AZ 85747
Phone 305 852 6121 Phone 800 423 2035
FAX 305 852 6272 FAX 520 298 8107


Appendix B

Sources for 8-2-12 “palm special” landscape fertilizer

Atlantic FEC Fertilizer Co.
18375 SW 260 st.
Homestead, FL 33031
Phone 800 432 3413

Howard’s Fertilizers
Contact: Erik Thor in Pompano Beach, FL
Phone 954 781 6944 (cell)

Nurserymens Sure Gro
4390 N. US 1
Vero Beach, FL 32967
Phone 561 770 0462 (office)
561 719 8968 (cell)




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