Genetic diversity among coconut varieties for susceptibility to Cape St Paul Wilt Disease

Abstract

The CSPWD is the Ghanaian form of lethal yellowing disease (LYD) of coconut, caused by a phytoplasma and has been active in Ghana since 1932. The paper updates the results of screening trials conducted with 38 pure and hybrid coconut varieties since 1981. Although no variety was found totally resistant, a wide range of susceptibility level was observed: almost all SGD were still unaffected, while the local WAT had almost totally disappeared, additive values were calculated for the parental varieties and it was shown that, in the average, hybrids are slightly more susceptible than predicted by a purely additive model. According to this genetic model, the SGD 9 VTT hybrid will be appreciably less susceptible than the MYD 9 VTT currently being used for replanting devastated farms in Ghana. Our results tend to confirm the general trend that cultivars from the Pacific group (especially the Dwarfs) are less susceptible than the Indo-Atlantic cultivars. Proposals are made to adapt planting material to the risk level. Genetic control can only be efficient if it is considered as a link in a chain of control measures involving the choice of a proper planting site, good management and early eradication of diseased trees.

Resistance screening trials on coconut varieties to Cape Saint Paul Wilt Disease in Ghana

“…64% in MYD × VTT plantings, but under the same conditions only 37% losses will be expected in the SGD × VTT [14]. The genetic modelling was found to show very good agreement between predicted and observed values and lends credence to the earlier assumption that the SGD × VTT should be a better alternative to MYD × VTT to replant devastated areas.…”

“…Situational reports of these trials have been published by Dery et al [9,13,14] and Mariau et al [12] and updated in this paper. It took variable period of field exposure to effect disease in the affected plots.…”

“…The Tumentu plot is still "disease-free" but the Cape Three Points plot was attacked in 2004 (9 years after planting). Situational report on this trial up to April 2006 has been published [14]. All three trial locations are active disease foci, and a computer generated completely randomized design was used to ensure uniform exposure of all varieties to infection; however, there is no CSPWD incidence as yet.…”

“…The hybrid was continuously exposed to CSPWD attack for 7 years, but after that, the disease became dormant in the plot. No incidence of CSPWD has occurred in this hybrid as yet; however, a "genetic modelling prediction" has designated it as a "promising hybrid" because of its expected low susceptibility to CSPWD [14].…”

“…However, in the 1981-83 plantings, MRD showed varied resistance levels at the different trial locations (table 3). A genetic modelling analysis of the results of the 1981-83 trials, determined MRD as one of the most resistant varieties so far [14]. The obvious question that arises is whether the highly susceptible and highly resistant MRD encountered in these trials are genetically identical?…”