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Which coconut palm for Lethal Yellowing in Ghana Report on the mission from 11 to 18 June 2006

Dollet M., Bonneau X., Baudouin L.. 2006. Montpellier : CIRAD-CP, 49 p.. numero_rapport: CP_SIC-1995.

The hybrid between the "Malayan Yellow Dwarf" and the "Vanuatu Tall" (MYD x VTT) was widely disseminated in Ghana. However, in the last 2 years, worrying Cape St Paul Wilt (CSPW ) percentages have been found on that hybrid. Although, in June 2006, we do not have enough hindsight regarding the performance of the hybrid between Sri-Lanka Green Dwarf (SGD) and the VTT in relation to CSPW, its performance can be inferred from that of its parents. Both seem to be much less attacked by CSPW than other varieties. However, in Ghana there are large differences in phenotype of the SGD palms according to local agro-ecological conditions. That is why it was felt to be very important to bring together specialists from different horizons, to examine and assess the following at the same time, at the same sites: the performance of the MYDxVTT hybrid, and those of the SGDxVTT. Between 1999 and 2004, 1 300 ha, involving 1 012 farmers in Central and Western Region, were planted with the MYDxVTT, through a grant awarded by CSDP- (funding by Agence Française pour /e Développement). We visited several of these plantations. The results are sometimes very disappointing. For instance, in Central Region at Elmina the disease arrived in August 2004 and so far at least 130 of the 160 MYDxVTT have been killed by CSPW, i.e. 81% mortality. Our observations made at Elmina and Asebu show that there can be some concern for the future of the 1 200 plots replanted with this hybrid. Whatever the case, the spread of CSPW leaves very little hope for farmers once the disease arrives in the plot. The new SGDxVTT seed garden in Bonsaso (2000/2001 - 10 ha), contains 2050 SGD palms. But a large part of it is in a hollow, the low-lying area of which is not drained despite recommendations made in 2002. There has been substantial mortality in this section. Five hundred replacements were made. Unfortunately, they have proved pointless, as most of them are in the bottom of the hollow and were standing in water at the time of the visit. In our view, there are 2 options for the future of this seed garden: 1- Either drain the hydromorphic zone; 2- Or abandon this zone and fell the palms planted there. With the 1 650 5GD palms producing, of which 1 000 are currently undergoing pollination, we can estimate a planting capacity of 200 ha of SGDxVTT hybrids per year from the end of 2007 onwards. At Agona Junction there is the only SGDxVTT demonstration plot in Africa and it is therefore important to study it properly! Should MYbxVTT be planted ? In the absence of the disease, this hybrid has very worthwhile agronomic potential. Under such conditions, it is possible to continue planting this hybrid, but only in "secured" zones, where inoculum pressure can be imagined to be zero or very weak. Our recommandation is: not to plant this hybrid within a radius of 5 km around an active focus (or even a diseased palm) and not before 1 year after the disease has passed. Should the SGDxVTT be planted? It can currently be said that it is most probably less susceptible to CSPW than the MYbxVTT. As the first agronomic estimations from Agona Junction are relatively satisfactory, it is a hybrid that we can recommend, but nonetheless avoiding, as a precaution, zones with a disease focus within a radius of 2.5 km, and ensuring phytosanitary monitoring in the surrounding zone. The CSPW transmission trials at Asebu gave an interesting result in 2005. The PCR checks on coconut palms exposed to different species revealed that one of the coconut palms exposed to Derbidae tested positive for the presence of CSPW phytoplasmas. But that palm had never displayed symptoms. In addition, another PCR analysis carried out 2 months later gave a negative result. Several hypotheses can be put forward: 1-The insects effectively "deposited" CSPW phytoplasmas "on" or "in" that coconut palm, but the phytoplasmas did no multiply; 2- The insects effectively transmitted phytoplasmas - They multiplied for a while, but the...

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