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Two mild strains of Cocoa swollen shoot virus (CSSV), N1 and SS365B, were assessed for their capability to mitigate the devastating effects of CSSV in field trials at the Cocoa Research Institute of Ghana in comparison with noninoculated cocoa plants. Analysis of growth, mortality, and yield of cocoa trees inoculated with the mild strains over a period of two decades between 1996 and 2014 indicated that the cocoa plants inoculated with mild strains performed significantly (P < 0.05) better in the presence of the severe strains (CSSV 1A) than noninoculated cocoa, albeit degenerated over time. The development of cocoa hybrids studied in the trial showed differences in their performance in the presence of the mild-strain viruses. Spread of the severe virus increased and deteriorated the effects of the mild isolates over time, thereby decreasing the economic life span of the cocoa trees. This observation was consistent in all the trials regardless of the type of mild strain involved. Apparently, the immunity and protection conferred by mild strains of CSSV (SS365B and N1) on cocoa declined over time, and we further confirmed ‘Amelonado’ cocoa to be very susceptible to CSSV infection in terms of effects on growth and yield.

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The spatiotemporal spread of cocoa swollen shoot virus disease (CSSVD), which is caused by cacao swollen shoot virus (CSSV) severe strain 1A in mixed hybrid cacao pre-inoculated with CSSV mild strain N1 (CSSV-N1), was investigated during a field experiment from 2006 to 2017, at the Cocoa Research Institute of Ghana. The development of disease epidemics has been described by the use of statistical modeling. Protecting all cacao plants with CSSV-N1 reduced the rate of CSSV-1A symptom appearance by 43% (P = 0.05) compared with the nonprotected control and by 33% compared with plots where cacao plants in the outer three or five rows were protected with CSSV-N1. Similarly, creating the protective outer rings three or five rows deep reduced the rate of CSSV-1A symptoms by 14% (P = 0.05) compared with the nonprotected control. CSSV-1A epidemics increased approximately 18% faster (P = 0.05) in transects oriented from the north and east compared with those oriented from the south and west. During the last 2 years of the study, CSSVD spread decreased significantly (P = 0.05) faster in plots where all test cacao plants were inoculated with CSSV-N1 compared with other treatments. The growth of cacao did not differ significantly among the treatments over the 9-year assessment period. Similarly, differences in the cumulative yield among the treatments over the 8-year assessment period were not significant.