Spatiotemporal Spread of Cacao Swollen Shoot Virus Severe Strain 1A in Mixed Hybrid Cacao Pre-inoculated With Mild Strain N1.

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.

Next generation sequencing elucidates cacao badnavirus diversity and reveals the existence of more than ten viral species.

Cacao swollen shoot virus is a member of the family Caulimoviridae, genus Badnavirus and is naturally transmitted to Theobroma cacao (L.) by several mealybug species. CSSV populations in West African countries are highly variable and genetically structured into several different groups based on the diversity in the first part of ORF3 which encodes the movement protein. To unravel the extent of isolate diversity and address the problems of low titer and mixed viral sequences in samples, we used Illumina MiSeq and HiSeq technology. We were able to reconstruct de novo 20 new complete genomes from cacao samples collected in the Cocoa Research Institute of Ghana (CRIG) Museum and from the field samples collected in Côte d'Ivoire or Ghana. Based on the 20% threshold of nucleotide divergence in the reverse transcriptase/ribonuclease H (RT/RNase H) region which denotes species demarcation, we conclude there exist seven new species associated with the cacao swollen shoot disease. These new species along with the three already described leads to ten, the total number of the complex of viral species associated with the disease. A sample from Sri Lanka exhibiting similar leaf symptomology to West African CSSD-affected plants was also included in the study and the corresponding sequence represents the genome of a new virus named cacao bacilliform SriLanka virus (CBSLV).

Use of Immune Crops as Barrier in the Management of Cacao Swollen Shoot Virus Disease (CSSVD)-Long-Term Assessment.

A field trial was conducted at the Cocoa Research Institute of Ghana from 1992 to 2004 to investigate the prospects of using Cacao swollen shoot virus (CSSV)-immune crops as a barrier to prevent the spread of the virus from existing outbreaks into newly established cacao plantings. The treatments consisted of four crops-citrus (Citrus spp.), oil palm (Elaeis guineensis Jacq.), kola (Cola nitida Vent.), and cacao (Theobroma cacao L.)-planted as a barrier between cacao trees serving as test plants on one side and as source of CSSV strain 1A infection on the other. Over a 7-year period post CSSV 1A inoculation, the citrus and oil palm barriers were the most effective in protecting test cacao trees from cacao swollen shoot virus disease (CSSVD) spread. The two crops gave comparable results (P = 0.9766) in terms of the cumulative number of visibly infected trees (1 and 5 of 522, respectively). The kola barrier, on the other hand, had a significantly (P < 0.000l) higher number of symptomatic trees (89 of 522) than citrus and oil palm. All three crops (citrus, oil palm, and kola) provided better protection than the control (cacao), which had 134 visibly infected trees out of 522. As shown by economic analyses, growing citrus, oil palm, or kola as barrier crops was as profitable as growing cacao. The implications of the results on the management of CSSVD in Ghana are discussed.

Appraisal of Cocoa swollen shoot virus (CSSV) Mild Isolates for Cross Protection of Cocoa Against Severe Strains in Ghana.

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.