Virus Detection and Elimination in Cocoa ( Theobroma cacao L.) Through Somatic Embryogenesis

Abstract

Cacao swollen shoot virus (CSSV) is a major pathogen that has seriously constrained cocoa production in West Africa, particularly Ghana and Nigeria. The objective of this study was to assess the efficacy of cocoa somatic embryogenesis to produce virus-free clonal propagation material both for replanting and to facilitate the safe international exchange of germplasm. Polymerase Chain Reaction (PCR)-based screening, is employed in this study because of its capacity for CSSV detection prior to the appearance of visual symptoms. Degenerate PCR primers were developed in order to improve the CSSV-strain dependence of earlier tests. The degenerate primers were capable of detecting 37 out of a putative 56 CSSV strains, four more than the sequence specific primers. For tissue culture studies, cocoa staminodes cultures were established from flowers of CSSV-infected cocoa genotypes CL 19/10 strain 1A and Amelonado Plant 2 to produce callus, primary and secondary somatic embryos, with genotype AMAZ 15 used as a virus-free control. PCR-based CSSV detection proved that virus could be detected at callus, primary somatic embryos and secondary somatic embryo stages, indicating that the progress of the virus was progressively impeded. These findings support the use of somatic embryogenesis as a mean of improving CSSV-free clonal propagation of cocoa. Somatic embryogenesis is indeed effective for virus elimination in cocoa and it has been demonstrated to function for a range of cocoa genotypes. This also means that a likely mechanism for the interruption of CSSV movement has been identified.