SCREENING FOR RESISTANCE TO LETHAL YELLOWING DISEASE OF COCONUTS IN GHANA: AN UPDATE OF TRIAL RESULTS

S.K. Dery¹, Y.P. N'Cho², R.N. Quaicoe¹, E.D. Arkhurst³, R. Philippe⁴

INTRODUCTION

The Coconut palm (Cocos nucifera) is a very important cash crop along the coastal belt of Ghana. It is estimated that Ghana produces a total of 224,000 tons of coconut annually from 43,000 hectares of land (Wadhwa & Kumar, 1992; Ofori & Nkansah-Poku, 1995). An important characteristic of coconut cultivation in Ghana is its ability to spread wealth and generate employment in rural areas where few other employment opportunities exist (Adams et al., 1996).

The most pressing problem facing the coconut industry in Ghana is a Lethal Yellowing-type disease called Cape St. Paul wilt disease (CSPWD). The disease has destroyed an estimated 11,500 ha of coconut in the three coconut-producing regions and is still spreading (Dery et al., 1995). Because coconut in Ghana is grown in small holdings (05 -5ha) (Owasu-Nipah & Dery, 1994), this destruction has adversely affected the livelihood of thousands of people. In desperation, some farmers have replanted their devastated fields with the highly susceptible West African Tall (WAT) variety.

The CSPWD is caused by a phytoplasma (Tymon et al., 1995) and is suspected to be transmitted by insects.

Given the low input subsistence nature of coconut cultivation in Ghana, the use of tolerant ecotypes is the most promising and economic rnethod of bringing the disease under control.

Screening for CSPWD tolerance in Ghana started in 1956 (Chona and Adansi, 1970). A more concerted effort at CSPWD resistance screening was started in 1981 with finding from the Government of France under the France/Ghana/Cote d'Ivoire Coconut project. Seven trial plots were set up at various sites in the Western Region (de Taffin & le Saint ,1987). A total of 27 ecotypes were involved. Under the EU-Science and Technology for Development (EC-STD) finding, 2 new plots were added. This brought the total number of ecotypes under test to 38.

SCREENING TRIALS

All the trial plots were planted in a complete randomised block design.

As at September 1999, 5 out of the total 9 plots have been affected by the disease: Cape Three Points, Akwidaa, Dixcove, Agona Junction, and Princess. Results of the Dixcove and Cape Three Points trials were given in Dery et al. (1995) and Mariau et al. (1996). At the time, the disease incidence in Akwidaa was very low (only 3 palms affected) and both Princess and Agona junction were disease-free. This paper presents the results as of September 1999 for the Agona Junction and Akwidaa trials. At Princess, only one palm has so far been attacked by CSPWD.

The Akwidaa trial was planted in 1981 in an already devastated area. At the time of planting the disease was no more active in the area. A total of 13 ecotypes were planted here.

The Agona Junction trial was planted in 1981-1983 in a non-diseased zone. Twenty-seven ecotypes were planted.

RESULTS

Akwidaa

The disease first appeared in this plot in l988 and, by 1990, three palms had been affected. The disease then disappeared from the plot until 1996 when two palms were infected again. It then started spreading steadily up to date.

Table 1 gives details of disease infection at both the Akwidaa and Agona Junction plots. At Akwidaa, the high yielding hybrid MYD x WAT and another hybrid SGD x WAT have the highest percent infection (69.57%). The next is CRD x WAT (66.67%) and then the control (WAT) (34.55%).

So far 37.21% of the total palms planted have been affected by CSPWD.

The SGD and MRD have not been attacked.

Agona Junction

A total of 39.56% of the palms planted at Agona Junction have been attacked by CSPWD.

The ecotype with the highest percent infection is again the MYD x WAT (91.30%). This is followed by SGD x WAT and MRD x WAT with 70.83% and 59.09% respectively. The MYD shows 58.33% infection. So far the SGD, VTT x VTT, MYD x VTT, MYD x RNT, CRD x RNT and MYD x PYT have not been affected.

DISCUSSION

These results need to be interpreted with caution as the number of palms is small and the disease is still active in the plots, but the results give some indication of which ecotypes might be tolerant.

The results again confirm the tolerance of the SGD and, to some extent, the VTT. The VTT was not planted at Akwidaa and Cape Three Points. (Dery et al., 1995). The hybrid MYD x VTT is also performing very well as it did at Dixcove. The MYD which has shown tolerance in the Caribbean has again been shown to be susceptible: 58.33% infection at Agona and 18.73% at Akwidaa so far. It has to be borne in mind that there are different strains of the Lethal Yellowing phytoplasma around the world (Tymon et al., 1995) so differences in tolerance can therefore occur.

Even though the observations are still on-going, the performance of the SGD is encouraging. It has shown 100% survival at Cape Three Points and Dixcove and is doing well at Agona and Akwidaa even under the unnatural environment replicated trials that often results in intense disease pressures on small numbers of palms. The case of the VTT is not quite clear because it is absent from the Akwidaa and Cape Three Points trials.

ACKNOWLEDGEMENT

The trials were set up with funding from tire Government of France (1981-1993). The EU-STD continued funding from 1993 - 1997 and the National Agricultural Research project (NARP) from 1997-1999. Their assistance is gratefully acknowledged.

REFERENCES

Adam, M., Arthur, R., Ghartey, N., Overfield, D. and Willougby, N. 1996. Analyses of natural resource utilisation, livelihood systems, coping strategies and natural resource research needs in coconut growing areas of coastal zones ecosystems in Ghana; Phase 1. N.R.I. Doc. C0834 Vol. 1, June 1996.

Chona, B. L. and Adansi, M.A. 1970. Coconut in Ghana. Crops Research Institute Bulletin No. 3. Council for Scientific and Industrial Research (CSIR) Kwadaso, Kumasi - GHANA.

Dery, S.K., N'Cho, Y.P., Sangare, A. and Arkhurst E.D. 1997. Cape St. Paul Wilt Disease resistance screening and prospects for rehabilitating the coconut industry in Ghana. Pp. 147- 152, In: Eden-Green, S.J. and Ofori, F. (eds). Proceedings of an international Workshop on Lethal Yellowing-Like diseases of coconut, Elmina, Ghana, November 1995. Natural Resources Institute, Chatham, UK.

Mariau, D., Dery, S.K., Sangare A., N'Cho Y.P. and Philippe R. 1996. Coconut lethal yellowing disease and planting material tolerance. Plantations, Recherche, Développement 3 (2): 105-110.

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Owasu-Nipah, J. and Dery, S.K. 1994. Coconut Breeding Programme in Ghana. Pp. 114-118, In: Batugal, P.A. and Ramanatha Rao V. (eds). Coconut Breeding. Paper presented at a Workshop on standardisation of coconut breeding research techniques, June 1941, Port Bouet, Cote d'Ivoire. IPGRI - APO, Serdang, Malaysia.

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Wadhwa, N.C. and Vijaya Kumar, CR.. 1992. Development of coconut products in Ghana: Project Proposal No GHA/87/004 of February 1992 (unpublished).