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Red alert to Lethal Yellow

Coconut palms are a vital part of the subsistence agriculture in many tropical coastal areas. The trees require very few inputs, and yet provide food, fuel and shelter as well as much-needed income. However, the relentless spread of a fatal disease of coconuts throughout the Caribbean region is having a serious impact on many vulnerable communities. And, even more worryingly, a new strain of the disease appears to be sweeping across Jamaica killing off dwarf and hybrid varieties that were thought to be resistant to the disease.

The 'telegraph pole' stage of the disease. Honduras.
credit: Dr. Maria Mercedes Roca de Doyle, Pan-American College of Agriculture, Zamorano, Honduras.

First described in Jamaica in the 1800s, coconut lethal yellowing disease (LY) is endemic on many Caribbean islands, and is caused by a mycoplasma (a bacteria like organism with no cell wall), which is transmitted by the sap-sucking planthopper, Myndus crudus. The disease reached the American mainland in 1956, spreading northwards killing not only coconuts but a number of ornamental palms which also proved susceptible to the disease. In 1980 it was recorded for the first time in Mexico and perhaps aided by natural disasters, such as hurricanes Hugo and Mitch, the LY epidemic continued to spread. By 1992 it had reached Belize and, in 1995, it was detected on Roatan, one of the Honduran Bay Islands and shortly after reached the Honduran mainland. Over 90% of Honduras' Atlantic coast palm trees have now been wiped out and the disease has since been recorded in Guatemala. In addition, there are unconfirmed reports that it has spread to Honduras' Pacific coast and into Nicaragua.

"Typical LY symptoms begin with a premature drop of most or all of the fruit," explains Dr. Phil Jones, a plant pathologist based at the Institute of Arable Crops Research, Rothamsted in the UK. "Older leaves then start to turn yellow and, eventually, all the leaves turn brown. By the time the apical bud dies, all parts of the tree are beginning to decay. Finally, three to six months after the advent of visible symptoms, the crown withers and topples from the palm leaving a bare trunk standing."

A lethal limit?

There is no effective cure for lethal yellowing although the disease can be effectively controlled using an oxytetracycline antibiotic formula injected into the trunk every 3-4 months. But costs are prohibitive for subsistence farmers. The long-term solution is to replant affected palm plantations using resistant coconut varieties, such as 'Malayan Dwarf.' However, these grow very slowly, taking 40 years to reach full maturity. The alternative is to plant the faster-growing Maypan hybrids, which were originally developed in Jamaica and are derived from a cross between the common but susceptible 'Atlantic Tall' and the resistant 'Malayan Dwarf'. However, these hybrids have to be produced in special seed gardens and can only be bought from limited suppliers. They are relatively expensive and, because of their hybrid nature, do not breed true to type.

In addition, many indigenous peoples such as the Garifuna, a race descended from Africans slaves in Honduras and Belize, prefer susceptible 'Atlantic Tall' palms. "We had a lot of problems with convincing the communities to adopt the new resistant varieties that are not as agronomically robust as the much-loved Atlantic Tall," says Dr. Maria Mercedes Roca de Doyle of the Pan-American College of Agriculture in Zamorano, Honduras. "Unlike Atlantic Talls, that grew wild, needed very little input and were seen as "God's gift", the resistant hybrids require irrigation and fertilisation, and the nuts they produce are not as desirable." Dr Roca De Doyle, along with other scientists and agencies, is involved with a programme known as 'Wafaluma', meaning 'Our Coconut' in the Garifuna language, to help tackle the disease in Garifuna smallholdings and, despite a slow start, 17% of the total affected area was replanted in 2001.

Early stages of the disease in ornamental palms. Florida, USA.
credit:Dr. D. Caldwell, University of Florida Extension Service, Florida, USA.

Until recently, the main source of healthy planting material was obtained from the Hacienda Victoria Seed Garden near Límon in LY-free Costa Rica. However, despite successfully marketing its certified disease-free coconuts throughout the Caribbean region, Richard Illingworth, the garden's executive director explains why the the seed garden is currently under threat. "Relatively high labour costs in Costa Rica, price inelasticity and slow acceptance of a new product in Central America, which had no history of paying for coconut seed, have rendered the project unprofitable over time. Although the seed garden is the only commercial regional supplier of LY-resistant certified hybrid coconut seed in quantities, hybrid production was suspended in October 2001 and a buyer is being currently sought.

A deadly dilemma

In the meantime, scientists are worried that the resource might be lost forever, and a delay in developing another could be critical to the lives of many of the region's coconut growers.And time could just be running out. A new kind of LY epidemic, with small genetic differences compared to other isolates, appears to be sweeping across Jamaica, killing off supposedly-resistant dwarf and hybrid varieties. Dr Roca de Doyle visited the island in January 2002 and reports that "What I saw sent shivers up my spine as I could see the same 'resistant' varieties that we are using for our replanting efforts in Honduras dying of this lethal yellow in Jamaica."

Article submitted by Peter McGrath, freelance journalist

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