The disease has already destroyed coconut plantations in several world regions: East Africa, the Caribbean and Central America. Like all known phytoplasma diseases, it is probably transmitted by an as yet unidentified insect. Attacks result in fruit fall and in frond yellowing and then frond fall. The palms die within a few months, leaving a field of bare stems.
In western and central Ghana, lethal yellowing disease has been wreaking havoc since 1964. Within around twenty years, more than 6 500 hectares were devastated. To date, just one variety, the "Sri Lanka Green Dwarf" seems to be resistant, but trials have shown that it does not adapt easily to the growing conditions in Ghana. The "Malayan Yellow Dwarf", however, has proved to adapt well in the past. Lastly, the "Vanuatu Tall" is less severely affected than the local "Tall" variety. There are thus plans to replant with hybrids between "Dwarf" and "Tall" varieties.
Based on observations made in the 1980s-90s, researchers began testing a hybrid in 1999, obtained by crossing the "Malayan Yellow Dwarf" variety with the "Vanuatu Tall". The trials were launched under a programme conducted by CIRAD in Ghana, from 1999 to 2004, as part of an operation to support the commodity chain, funded by the Agence française de développement. In all, almost 210 000 plants were distributed to more than a thousand growers, who replanted 1 300 hectares of coconut plantings.
Unfortunately, the results were not convincing. In zones where the disease is very active, the hybrid was affected as soon as the second year. Moreover, the "Vanuatu Tall" has no longer been resistant to the disease since 2003. In the centre of the country, one-hectare plots have already suffered substantial losses: between 19 and 80% in less than five years. Although a large number of plots are still unaffected today, once the disease occurs, there is very little chance for smallholders.
Moreover, the trees need to grow correctly from the outset. Again, the results have been mixed: two thirds of the farms observed during the project obtained growth rates similar to those of reference plantations, or showed only a slight delay. However, the remainder fell well short of the standard growth models. There are many reasons for this variability (soil fertility, water stress, cropping practices, pressure from insect pests, etc).
The strategy in future will consist in only distributing the hybrid to areas with low parasite pressure. In central Ghana, where that pressure is high, a new variety is due to be tested: a cross between the "Sri Lanka Green Dwarf" and the "Vanuatu Tall". Moreover, a project financed by the Fonds de solidarité prioritaire (FSP) has just been luanched. It is managed by the Cooperation and Cultural Aid Service at the French Embassy in Ghana, and in particular includes a study of the pathogen, its variability and its diagnosis, a "vector determination" component and another component on "agro-socioeconomics", which includes a study of farmers' strategies and practices in response to the disease.
Helen Burford | alfa
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