Vegetative buds of three cocoa varieties,' Amelonado' (P30), 'Trinitario' (K5) and 'Upper Amazon' (T85/799) were irradiated with 15, 20 and 25 Gy of y-rays, respectively, and budded on to rootstocks to generate MVl shoots. The terminal buds of the shoots were removed to induce the formation of MV2 shoots, from which MV3 shoots were similarly derived. The MV3 plants were screened for resistance to the Cocoa Swollen Shoot Virus (CSSV) by patch-graft inoculation of the rootstocks. Only a few plants from each of the three cocoa varieties were found to be symptomless after indexing. These plants were multipUed by budding to the MV4 and MV5 stages and screened at each stage for CSSV resistance by inoculation using virus-carrying mealybugs. At the MV5 stage, some plants still remained symptomless and this was confirmed with enzyme-Unked immunosorbent assay (ELISA). The study reveals that y-rays can be used to induce genetic variability for resistance to CSSV in cocoa, as well as for other traits such as chlorophyll deficiency.Key words: Theobroma cacao -cocoa swollen shoot virus (CSSV) -mutagenesis-ELISA testThe cocoa swollen shoot virus disease (CSSVD) is of great economic importance in the West African sub-region (Thresh 1980) where the bulk of the world's cocoa is currently produced. Earlier attempts to control the vector mealybugs Planococcoides njalensis (Laing) (Homoptera: pseudococcidae) by chemicals (Hannah 1954, Hannah et al, 1955, Bowman and Casida 1958 and by biological control (Donald 1953, Decker 1955 failed. An eradication method which entails the removal of infected trees from their symptomless neighbours (Posnette 1943) initially appeared successful, but despite the removal of over 190 milUon infected trees (OUennu and Hughes 1990) the disease persists. Breeding for resistance to the disease appears to be a plausible approach to combat it but earlier breeding programmes were also largely unsuccessful, mainly because of the narrow genetic base available at the time. Some resistant genotypes were developed (Legg and Lockwood 1975) from largescale introductions from the centre of diversity (the Amazon Basin), but crop losses are still very high (Ollennu et al. 1989). Better sources of resistance to the disease are therefore needed and more introductions from the centre of diversification and elsewhere are being used. However, as CSSVD is not found in the centre of diversification of the plant (i.e. the Amazon Basiti), the sources of stronger resistance to the disease might not be available from there. It is with this in view that this mutation breeding programme was undertaken with the objective of inducing CSSVD-resistant mutants that could be used to develop improved varieties.Plagiotropic vegetative budwood were collected from three types of cocoa {Theobroma cacao L.), a local 'Trinitario' (K5). 'Upper Amazon' (T85/799) and a local 'Amelonado' (P30). These varieties have most of the good quaUties of cocoa, such as tree-to-tree uniformity, large bean sizes and high bean numbers per pod in addition to good...
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