19891 HOWARDAND BARRANT:LETHAL YELLOWINGS t63 Principes,33(4), 1989, pp. 163 171 Questionsand Answersabout Lethal Yellowing Disease F. W. HoweRD ANDC. I. Be.nnexr Uniuersity of Florida, Fort Lauderdale Research artd Education Center, 32O5 CoIIege Auenue, Fort Lauderdale, FL 33314 and Coconut Industry Board, P.O. Box 204, Kingston 10, lamaica, West Indies Lethal yellowing (LY) diseaseis one of the disease is comparatively limited at the most serious threats to palms world- present. wide becauseit is fast-spreading,kills palms The careers of most of the members of "LY rapidly once they are infected, incurable the original Florida and Jamaica at present, and affects many speciesof researchteams" havetaken them separate palms. Thanks partly to some membersof ways. Having been associated'with the the International Palm Society, a research original LY research teams, .the authors program with a multidisciplinary team of continue to conduct research on LY as a six scientistsrepresenting the fieldsof plant part of the Ft. LauderdaleCenter's long- pathology,entomology and horticulture was term interest in tropical ornamental hor- establishedby the State of Florida in the ticulture and the Coconut Industry Board's 1970's at the University of Florida'sFort mission to serve the coconut industry of Lauderdale Researchand Education Cen- Jamaica. ter to try to find solutionsto this problem. Over the years,we have receivedmany The Fort Lauderdalegroup soon formed a questions on LY from growers, horticul- close working relationship with a similar turists, and others interestedin palms. The LY researchgroup which had been orga- most frequent and some of the most inter- nized in the Research Department of the esting questionsare presentedhere. Coconut Industry Board of Jamaicaby the British OverseesDevelopment Ministry. By the early l9B0's, due to changesin Wich palm speciesare rnost suscep- budgetary priorities and other administra- Q. tible to LW tive considerations,both LY researchpro- gramswere phasedout by their respective A. The coconutpalm (Cocosnucifera) and. governments. This was unfortunate. speciesof Pritchardia are the most sus- Together, the two groups had developed ceptible species(Howard et al. 1979). more knowledgeof the basic nature of the However, there are different degrees of disease,as well as practical ways to man- susceptibilityamong the many varieties of 'Jamaica age it, than had been accumulatedin the coconutpalm. The Tall' variety, 100 years or more since LY had been which is the most common coconut palm known in the Caribbean Region. Lethal seenthroughout the CaribbeanRegion and yellowing is still with us, and in fact, has formerly in southeasternFlorida and the spread into new localities in the last few Keys, is one of the varieties that is highly years,but the momentumof theseresearch susceptibleto LY (Harries 1971, 1974). programs has been lost and researchon In Jamaica,where this was once the prin- r64 PRINCIPES [Vor. 33 I. A plantation in Jamaica of'Jamaica Tall' coconut palms destroyed by lethal yellowing disease. cipal variety grown, entire plantationswere associatedwith certain plant diseases.Since lost to LY within periods of a year or two it has not been definitely establishedthat (Fig. l). There are more than 30 species they are mycoplasmas,they are called known to be susceptibleto LY (Table l). mycoplasmallfte organisms. o'catch" Q. What causesLm Q. How do palms LY diseo.se? A. Scientific studies have indicated that A. The mycoplasmalikeorganisms which LY is causedby mycoplasmalikeorganisms causeLY are transmittedby the American (\4LOs).Mycoplasmas are microorganisms palm cixiid, Myndus crudus. (A cixiid is that are intermediatebetween viruses and a planthopper of the family Cixiidae.)This bacteria.They have beenknown for some informationis basedon fieldevidence (e.g., time as pathogensthat causecertain dis- Howardl9B0) andon experimentsin which easesof animals.In the last two decades, American palm cixiids were collectedfrom similar oreanisms have been found to be palmsin areaswhere the diseasewas active r98el HOWARDAND BARRANT:LETHAL YELLOWINGS 165 and introducedinto cagescontaining palms Table 1. List of palm taxa susceptible previouslyunexposed to the disease.Palms to lethal yellowing in Florida, and rel- in caeesinto which the insects were intro- atioe susceptibility, Iune 1989. duceJ contracted LY. Palms in similar cageskept free of these insects remained Relative Susceptibilityl healthy(Howard et al. l9B3). Further test- ing wasdone with youngerpalms and palms Aiphanes lindeniana unknown unknown of different species with similar results Allagoptera arenaria Arenga engleri 3 (Howardet al. 1984). Borassus f.abellifer 2 Caryota mitis 2 pus cabadae I Arner- Chrysalidocar Q. Are there insectsother than the Cocos nucifera 3 ican palm cixiid that could spread LY? Corypha elata 3 Dictyosperma album 2 A. Our studiesindicate that the American Gaussia attenuata unknown palm cixiid is the principal and possibly Houea belmoreana unknown the only vector of LY in the Americas. If Hyophorbe aerschafeltii 2 there were other insect speciescapable of Latania spp. 2 I they would be Liuistona chinensis transmitting the disease, Nannorrhops ritchiana unknown limited to Iocalizedareas. In most casesin Neodypsis decaryi unknown which diseasesare spread by more than Phoenix canariensis 2 one speciesof insect, all of the speciesare P. dactylifera 3.- I taxonomicallyclosely related. Thus, if there P. reclinata P. rupicola unknown is a vector of LY additional to the Amer- P. syluestris unknown ican palm cixiid, we would expect it to be Pritchardia spp. 3 another speciesof cixiid. Except for the Rauenea hildebrandtii unknown American palm cixiid, speciesof this group Syagrus schizophylla 2 2 Caribbean Trachycarpus fortunei are quite rare on palms in the Veitchia arecina unknown Region. In conclusion,if we could control V. merrillii 2 the American palm cixiid, we could control V. montgomeryana 2 LY in most (if not all) placesin the Amer- Veitchia sp. (Sunshinepalm) 2 icas where the diseaseis present. I Susceptibility ratings based on combined obser- vations of Fort Lauderdale Research and Education Center personnel. I : slightly susceptible,2 : mod- Q. Where did LY comefrom? erately susceptible,3 : highly susceptible. A. Lethal yellowing may have originated in the Western Caribbeanarea, as there reports of what appearsto have been were In which countries has LY been in coconutpalms in the Cay- Q. this disease reported? man Islandsas early as 1832, and in Cuba, Jamaicaand Haiti in the late lB00's. Palm A. In the'WesternCaribbean Region: Cuba, speciesnative to the Americas,particularly Jamaica, the Cayman Islands, Hispaniola to the CaribbeanRegion, are almost never (Haiti and the Dominican Republic), New affected by this disease, suggesting that Providence(Bahamas), Yucatan and Quin- perhapsin past ages the diseasespread tana Roo (Mexico);in the United States: repeatedlythrough this region, thus select- Florida and Texas.In Tropical West Africa, ing out resistant strains of these palms a similar or perhaps identical diseasehas (reviewedby Howard 1983). There are beenreported in Ghana,Togo, Cameroon alsoreasons to suspectthat LY may have and Nigeria. Also, in Tanzania(East Africa) evolved in Asia (Harries I979). an LYlike diseaseis beine studied. Since I66 PRINCIPES [Vor. 33 no major differenceshave been detected Q. Wat is the current situation regard- between the diseasesof Africa and the ing LY in Jamaica? Americas, they are all referred to as LY A. In Jamaicathere were formerly an esti- (reviewedby Howard 1983). 'Jamaica mated 5 million Tall' coconut palms(Harries 1974). LY destroyedmost Q. What is the current distribution of LY of these by the l980's, and they were in Florid.a (Iune of 1989) ? replacedby'Malayan Dwarf palms.There 'Jamaica are still,some Tall' coconut A. Key'West was where LY was first dis- palms throughout the island, and they continue covered in Florida, possibly as early as to be eliminated by LY. 1937. Between1955 and 1960, an epi demic of LY eliminated about 15,000 (75%) of the coconutpalms in Key West Q. Since LY is spread.by an insect, can (Martinez and Roberts 1967). Today, a the diseasebe controlled by spraying the visitor travelling the OverseasHighway to palms with an insecticid.e? Key West will passover many islandswith abundant standsof healthy coconut palms A. Although the American palm cixiid can 'Malayan 'Jamaica (the Dwarf and Tall' be controlled by insecticideson a limited varieties and hybrids between these are basis (Reinert I977), insecticidalcontrol represented)but may occasionally spot a of the vector is not promising as a long- palm or perhaps a cluster of palms with range strategyto control this disease.We LY symptoms. Coconut palms are still \ryereable to suppresspopulations of this common on Key West, and some of the insect and bring about a reduction of 50 'Jamaica old Tall' palms (e.g., near the to 7 5% in the rate of spreadof LY in large beach on the south side of the island) sur- experimental blocks by spraying palms vived the earlier LY epidemics. Some biweeklyfor l4 months.A 50% reduction islands,e.g., ConchKey and figeon Key, in the spread means that it would take have remained virtually free of LY. twice as long for all of the palms to die. The epidemicof the 1970's and early Perhapsthe rate of spreadcould be slowed l980's on the east coast from Miami to further by more frequent applications of the Palm Beaches(Howard 1980) hassub- more toxic and more persistentinsecticides sided because few palms of susceptible to even larger blocks of palms, but large speciesare left. LY continuesto spread scaleuse of insecticidesto control this dis- among these widely separated susceptible ease would entail unacceptable costs and palms. (The extensiveplantings of coconut environmental hazards (Howard and palms along beachesare notable excep- McCoy I9B0; Howard,unpublished data). tions.) During the 1970's there were rel- atively few casesof LY on the west coast How was it d,eterminedthat LY is of Florida or on the east coast north of Q. caused by MLOs? Jupiter Inlet (Howard 1980). However, within the last few years there has been a A.