Cacao Swollen Shoot Viruses in Ghana

A barcode database for insects associated with the spread of the Cocoa Swollen Shoot Virus Disease in Côte d'Ivoire

Swollen Shoot is a viral disease affecting cocoa trees, transmitted by several species of mealybugs (Insecta, Hemiptera, Sternorrhyncha, Pseudococcidae). These insects maintain trophobiotic relationships with a complex and species-rich assemblage of ants protecting them and natural enemies controlling their populations. Here, we provide a curated DNA barcode database to characterise this insect community. Systematic observation of 7,500 cocoa trees was conducted, coupled with the collection of mealybug colonies and associated insect communities (parasitoids, predators and ants). Natural enemies were reared from mealybug colonies collected from 1,430 cocoa trees. Specimens were identified morphologically and sequenced for fragments of the standard DNA barcode region of the COI. We recovered 17 species of mealybugs from the family Pseudococcidae. Amongst these species, eight are new to the Ivorian cocoa orchard: Dysmicoccusneobrevipes Beardsley, Ferrisiadasylirii (Cockerell), Maconellicoccusugandae (Laing), Paracoccusmarginatus Williams & Granara de Willink, Phenacoccussolenopsis Tinsley, Planococcusminor (Maskell), Pseudococcusconcavocerarii James and Pseudococcusocciduus De Lotto. Three of these species were identified for the first time in cocoa orchards in Africa: D.neobrevipes, Fe.dasylirii and Ph.solenopsis. A total of 54 ant species were identified and represented the first record of these species associated with mealybug colonies in cocoa in Côte d'Ivoire. Amongst the species associated with the mealybugs, 22 primary parasitoids, eight hyperparasitoids, 11 ladybirds beetles (Coccinellidae), seven gall midges (Cecidomyidae), one predatory lepidopteran species and four spider species were identified. Nine species of mealybugs parasitoids are newly recorded in the African cocoa orchards: Acerophagusaff.dysmicocci, Aloencyrtus sp., Anagyruskamali, Anagyrusaff.pseudococci, Aenasiusadvena, Clauseniaaff.corrugata, Gyranusoideaaff.tebygi, Zaplatycerusaff.natalensis (Encyrtidae) and Coccophaguspulvinariae (Aphelinidae) and one hyperparasitoid, Pachyneuronmuscarum (Pteromalidae). For Côte d'Ivoire in particular, besides the previously mentioned nine parasitoids and one hyperparasitoid, five additional species are recorded for the first time, including four primary parasitoids, Blepyrusinsularis (Encyrtidae), Clauseniacorrugata (Encyrtidae), Clausenia sp. (Encyrtidae), and Coccidoctonuspseudococci (Encyrtidae) and one hyperparasitoid, Cheiloneuruscyanonotus (Encyrtidae). These results significantly enhance the knowledge of the diversity of the entomofauna associated with Swollen Shoot disease and pave the way for developing control methods based on the natural regulation of its mealybug (Pseudococcidae) vectors.

Molecular Surveillance, Prevalence, and Distribution of Cacao Infecting Badnavirus Species in Côte d'Ivoire and Ghana

The cacao swollen shoot disease (CSSD) caused by a complex of badnavirus species presents a major challenge for cacao production in West Africa, especially Ghana and Côte d'Ivoire. In this study, CSSD species detection efficiency, diversity, and geographic distribution patterns in cacao plantations in Ghana and Côte d'Ivoire were investigated through field surveillance, PCR detection assays, sequencing of positive amplicons, and phylogeographic clustering. Cumulatively, the detection efficiency of the tested CSSD primer sets that were targeting the movement protein domain of the virus ranged from 0.15% (CSSD-3 primer) to 66.91% (CSSD-1 primer) on all the symptomatic cacao leaf samples assessed. The identified CSSD species differed phylogenetically and overlapped in distribution, with the cacao swollen shoot Togo B virus (CSSTBV) (n = 588 sequences) being the most prevalent and widely distributed compared to the other CSSD species that were encountered in both countries. Geographically, the cacao swollen shoot CE virus (CSSCEV) species (n = 124 sequences) that was identified was largely restricted to the bordering regions of Ghana and Côte d'Ivoire. These results provide updated knowledge of the geographic distribution of the key CSSD species and their diagnostic efficiency and, thus, provide guidance in identifying locations for structured testing of cacao germplasm and optimal diagnostics for the predominant CSSD species in Ghana and Côte d'Ivoire.

Epidemiology and Diagnostics of Cacao Swollen Shoot Disease in Ghana: Past Research Achievements and Knowledge Gaps to Guide Future Research

Cacao swollen shoot disease (CSSD) caused by complexes of cacao swollen shoot badnaviruses (family Caulimoviridae, genus Badnavirus) remains highly prevalent and devastating in West Africa. The disease continues to impact substantially on cacao yield loss, cacao tree mortality, and decline in foreign exchange income from cacao bean sales. Currently, the disease is estimated to have a prevalence rate of over 30% in Ghana, as assessed in the ongoing third country-wide surveillance program. Although achievements from past research interventions have greatly elucidated the etiology, biology, epidemiology, diagnostics, and management of the disease, there are some outstanding knowledge gaps. The role of these information gaps and their effect on CSSD epidemiology and prevalence remain unanswered. This paper summarizes existing scientific knowledge from past research achievements that have provided elucidation on CSSD epidemiology, management options, and guided future research. The discussion highlights the need for multidisciplinary research with modern tools and institutional collaborators to holistically bring clarity on knowledge gaps on pathogen biology, virus-host--vector interactions, role of environmental and soil nutrient effects on CSSD severity, evolution pattern, role of alternative hosts on virus species diversity, vector population dynamics, and their overall impact on CSSD prevalence and integrated management in cacao plantations.