Studies on the Genetics of Cocoa Swollen Shoot Virus Disease (Cssvd) Resistance in Cocoa (Theobroma Cacao L.)

Abstract

Five Upper Amazons cocoa clones, namely NA33, PA150, PA7, SCA6 and SCA9 were selected for this study. The objective of the study was to ascertain the resistance level of the resistance to the CSSV disease. Test parents were clonally multiplied and inoculated and the expressed leaf symptoms studied. The difference observed in the parents were used to initiate hybridization. Mentor pollen technique was used to induce self-fertilization. Clonal multiplication was done by budding and mealybugs (P. njalensis) was used for the inoculation. Herrania balaensis was also used as mentor pollen to induce self-fertilization in self incompatible clones. Diallel crosses show that all genetic parameters studied are under polygenic control and all show additive effect. Reistance genes were found to be recessive to susceptibility. The order of resistance among the five clones was NA33 > PA150 > PA7 > SCA6 > SCA9. Comparative studies of the progenies of these crosses revealed that crosses between NA33 as female parent with PA7 and PA150 as pollen parent scored high percentages of resistance. Crosses between PA7 and PA150 also recorded high percentages of resistance. NA33 had higher general combining ability than the rest. The expressed leaf symptoms also vary from one clone to the other. The highly resistant ones either showed no symptoms or expressed symptoms associated with mild strain of the virus, while the susceptible ones showed some amount of virulent strain symptoms in addition to the mild ones. The General Combining Ability (GCA) and Specific Combining Ability (SCA) values for resistance were not significant but the GCA mean square was higher in the analysis than the SCA. The narrow sense heritability was high (79%),'