Lethal Yellowing disease is threatening coconut palms in Honduras and other parts of Central America and the Caribbean. This article focuses on the Honduran situation, explores the economic impact of the disease and reviews research efforts to manage the crisis. It suggests that biotechnology options to control LY could be more fully explored if adequate funds were available.

By nearly all measures of development, Honduras is one of the poorest countries in the Western Hemisphere. style="mso-spacerun: yes">  Seventy per cent of its 6 million inhabitants live below the poverty line, and infant mortality is 40.8 deaths per 1000 live births. Gross domestic product per capita is US$ 673, and malnutrition and lack of education continue to undermine the country’s human potential (FAO 1999). The Honduran economy, devastated by Hurricane Mitch in October 1998, also confronts a disease that threatens not only the country’s vital tourist industry but also the livelihoods of its coastal peoples. Lethal Yellowing (LY) disease has invaded the coconut palms that fringe the beaches of the Atlantic Coast and the Bay Islands and is spreading rapidly.

Lethal Yellowing disease

Honduras has the dubious honour of being at the epicentre of the current LY epidemic. Once the disease has entered an area, landscapes are changed in a matter of months. Beaches and plantations affected by LY resemble bizarre stands of blackened telephone poles in the aftermath of an explosion – an image that contrasts sharply with the tropical beaches displayed in the average tourist brochure.

The disease is not new, and there are reports of LY that date back to the 19th century in the Grand Caiman Islands. International attention was first drawn to the problem in the 1960s. By 1980, the disease had been responsible for the death of over 7 million palms in Jamaica. The Jamaican epidemic was matched by similar outbreaks in Cuba, Haiti, the Dominican Republic, the Bahamas and Florida. In 1977, the disease arrived in Cuzumel-Cancun in Mexico and began to move down the Yucatan Peninsula to Belize.

LY reached Honduras in 1995 and in the last six years has destroyed 70 per cent of the country’s original coconut population. It is expected to kill 90 per cent of the popular yet susceptible Atlantic Tall variety, a palm that is particularly common on the Atlantic coast and Bay Islands. If current trends continue, LY will probably spread southwards to Nicaragua, Costa Rica and Panama, before reaching South America. Lethal diseases of coconut are not confined to the Americas, and infective agents closely related to the LY pathogen are also causing havoc in coconut plantations in Africa and Asia.

Symptoms and impacts

In an affected palm, LY causes premature nut fall, yellowing foliage and necrosis of the inflorescence. Within six to nine months after these symptoms first appear, the palm is completely defoliated and dies. In Honduras, LY is not only having a serious impact on the aesthetics and biodiversity of its beaches and coastal areas, it is also threatening the culture and food security of local coastal peoples, especially the Garifunas, an ethnic group of African origin and the Misquito Indians. Other affected groups include communities of the Bay Islands.

Many local groups rely on palms and coconuts for their food, building materials and other household needs. The Garifuna and Misquito communities are already under considerable pressure. Pollution is rapidly degrading the environment on which they depend. The coral reef is gradually being destroyed, coastal waters are over-fished and the narcotics trade is seriously disrupting community life and culture. Like many other poor and vulnerable coastal communities, they were particularly hard hit by Hurricane Mitch.

Epidemiology

Despite research, there is still much speculation about how LY reached Honduras in the first place. It is possible that the tiny disease vector was simply swept by strong wind currents from other affected areas. But since the first reports of the disease in the Bay Islands area coincided with the hotel boom, some believe it was introduced by the tourist industry. The vector spends the immature stages of its life cycle in ornamental grass, and when this was imported into the region LY came too.

It is interesting that in Cayos Cochinos, a small group of islands that are protected and off-limits to tourist development, the disease did not appear until several years later even though they are close to the tourist centres of Roatan and Utila, other Bay Islands and continental Honduras. Although purely anecdotal, it is also noteworthy that the LY epidemic in Mexico also coincided with the development of tourism. There is no doubt, however, that Hurricane Mitch accelerated the spread of the disease by subjecting palms to mechanical and abiotic stresses, thus rendering them more susceptible to infection. It also exacerbated the dispersal of the vector.

In June 1999, the Pan-American School of Agriculture (Zamorano) joined forces with the Honduran Foundation for Agricultural Research (FHIA), the Honduran Government, and several NGOs to develop a programme to respond to the damage caused by LY. By creating a multidisciplinary group of social scientists, agronomists, plant protection and biotechnology specialists, a network known as Save the Coconut or Wafaluma in the Garifuna language, was set up in Honduras. It aimed to join forces with a much wider international group working towards a better understanding of the disease.

LY is often thought of as the “dengue of palm trees” because it is caused by a microorganism – a phytoplasma or mycoplasma – which is similar to a bacterium but has no cell wall. LY is transmitted by an insect – the planthopper Myndus crudus.

Phytoplasmas cause systemic infections. Electron microscopy analysis of tissue from affected palms reveals phloem cells packed with the phytoplasma. This is assumed to cause a physical obstruction to the flow of nutrients, which eventually kills the tree. Although there is still much to learn about the epidemiology of the disease, we know that not all members of the Palmacea family are susceptible to LY, and indeed not all varieties of Coccos nucifera (coconut palm) succumb to the disease. This is the key to LY management. For the region’s oil palm industry, it is reassuring that the oil palm is resistant to LY.

Not "all that is yellow" is LY, however. Several other diseases caused by fungi, nematodes and even flagellates can cause similar symptoms and these can be mistaken for LY. Since disease management strategies are vastly different for each disease, accurate diagnosis systems are vital. LY, unlike more mainstream fungi and bacteria, is notoriously difficult to isolate and visualize, and requires DNA-based molecular techniques for diagnosis and research. In 1998, the Zamorano team in collaboration with scientists from Europe, the USA, and Mexico established a molecular laboratory to diagnose and conduct applied research on the disease. The laboratory was also intended to serve as a reference laboratory for the region. This small, applied research and extension project has been supported by public funds from the USA and European donor agencies.

There is no known cure for LY, and vector control is difficult. Antibiotic applications of oxytetracycline could be used to suppress symptoms, but single palm applications are expensive and not suitable for large-scale use. The planthopper M. crudus, a known insect vector of the disease, has a feeding preference for the upper fronds of tall, mature palms. It ingests the pathogen while feeding on a diseased palm and infects healthy palms by injecting pathogen-containing saliva when taking its next feed. Vector control in this case would require aerial applications of toxic insecticides over large areas including beaches and protected areas. Its efficacy would depend on ensuring that the insecticide came into direct contact with the insect – a difficult task when the tiny planthopper hides beneath fronds swaying 30 meters above ground level.

The only viable solution lies in replanting affected areas with resistant varieties. This is a monumental task, since millions of palms would have to be replaced to protect the livelihood of Honduran coastal communities and restore the beaches of the Atlantic coast to their former state.

During the LY epidemic in Jamaica in the 1970s and 1980s, resistant varieties were identified and other resistant hybrids developed. Of these, the Malayan Dwarf varieties and the Maypan hybrid (a cross between the Malayan dwarf and a Panama Tall variety) were subsequently used in replanting programmes across the region. The ubiquitous Malayan Dwarf, although highly resistant to the disease, is not much liked by the coastal communities. They complain it produces inferior copra (dry coconut meat), has less taste and is not as robust agronomically as the highly susceptible Atlantic Tall variety. The Maypan hybrid is not popular either mainly because seedling production costs are high and the progeny cannot be used for replanting plantations though they could probably be propagated in Garifuna communities.

Applied research and appropriate solutions

From the 1960s to the early 1980s there was a period of relatively intense LY research conducted mainly by public institutions. The LY epidemic in Jamaica in the 1970s brought the local coconut oil industry into jeopardy and focused international attention on the seriousness of the disease. With the support of the British government, immediate attempts were made to develop disease management strategies. Research focused on isolating the insect vector(s) and finding and developing resistant varieties through conventional plant breeding. In the 1970s the USA also became involved when there was an outbreak of the disease in Florida.

Thirty years after research efforts began in Jamaica, there are again reports of an outbreak of LY in the resistant Malayan Dwarf and hybrid plantations. This raises the spectre that different strains of the pathogen can, over time, break the resistance of the current pool of useful varieties.

The Centre for Information on Coconut Lethal Yellowing (CICLY), an electronic discussion forum for anyone who is interested in LY, has pointed out the serious implications of the recent Jamaican outbreak. Research is needed to establish whether new and more virulent strains of the LY pathogen have become active or whether more efficient vectors have emerged.   Environmental and economic factors that have possibly contributed to the new outbreak need to be analysed and the steps that should be taken by farmers and institutions involved in rehabilitation programmes need to be identified.

It is clear that research into LY needs to be multidisciplinary and participatory and involve not only agronomists, geneticists, plant breeders and entomologists but also farmers, agricultural economists and rural sociologists. Such research is not cheap. Much basic work needs to be carried out with different palm and coconut varieties. At this stage, coconut plant diseases have not even been fully identified. Artificial culture of the pathogen or inoculation is not possible. Little is known about the vector(s) and even the evidence implicating Myndus in the transmission of the coconut phytoplasmas is limited. More research is also required to establish whether, for example, vector populations have changed as a result of their exposure to different coconut germplasm or to changes in the growing environments.

The implications of the current outbreak extend beyond Jamaica and have regional and possibly global consequences. Other countries in Central America and the Caribbean with LY problems, such as Belize, Honduras and Mexico, for example, need to know whether they should continue to plant varieties and hybrids that are considered to be resistant and what integrated pest control measures to implement. A concerted international effort is required to answer these pressing questions. However, the outbreak comes at a time when research funding is drying up. In Honduras and in many other parts of the world research funding for coconut is fragmented, severely under-funded and under-staffed.

A social component

New resistant varieties well adapted to local conditions and requirements are urgently needed, and wide-scale replanting programmes need to be found. However, three decades after work in Jamaica began, there is no meaningful funding either from public institutions in the affected countries or from donor agencies in the industrialized world to support the continuation of plant breeding programmes.

Developing resistant varieties is a lengthy process and takes years using conventional methods. The process could be speeded up with the help of biotechnology. Molecular markers, for example, could be used to detect possible resistant genes in varieties that have not yet been tested. Marker assisted breeding has been used successfully for diseases in other tree species, but no resistance markers have yet been published for coconut palms.

Biotechnology could also hold the key to the mass production of resistant varieties by tissue culture. This field has attracted some interest, but has much more potential. Studies using DNA-based techniques are already being carried out in several countries including Honduras, on the possibility of pathogen variability. Such attempts to understand the recent outbreaks of LY in resistant varieties need adequate funding so other scientists in the region can become involved.

Although still controversial and unexplored, there is also the issue of constructing a transgenic coconut variety that is resistant to LY and that has the agronomic characteristics of the highly prized Atlantic Tall variety. The relevance and consequences of genetic engineering and the heavy investment it involves for a country struggling to overcome poverty and malnutrition is a matter for national public debate. All countries in the region, including Honduras, have biosafety guidelines to regulate, at least in theory, the experimentation and planting of genetically modified crops. Most scientists, however, remain suspicious of allowing significant incursion of genetically modified material into the region for production purposes.

Whatever technological solution is developed, the success of any initiative will depend on an understanding of local culture and livelihood systems and the degree to which local communities are involved in the process. The current initiatives to create stronger alliances between scientists, development practitioners and local people in order to identify appropriate solutions are an important first step. A second is to stimulate funding for a coconut research agenda that directly addresses the needs of the local economy and an optimal use of the technologies available.

Pan American College of Agriculture, Zamorano, Honduras, P.O. Box 93, Tegucigalpa, Honduras.
Phone (+504) 776 6140/50 ext. 2362; Fax (+504) 776 6242; E-mail Mmroca@zamorano.edu.hn

Sources
Harries, H. (2000), CICLY Digest Centre for Information on Coconut Lethal Yellowing. No. 147.

Smith, R. (2000), CICLY Digest Centre for Information on Coconut Lethal Yellowing. No. 506.

Manicad, G. (1995), "Technical changes and the perils of commodity production: Biotechnology and the Philippine coconut farmer" Biotechnology and Development Monitor, No. 23, pp 7-10

Centre for Information on Coconut Lethal Yellowing (CICLY), electronic discussion forum. http://www.yahoogroups.com/messages/cicly