The cacao swollen shoot virus disease is among the most economically damaging diseases of cacao trees and accounts for almost 15-50% of harvest losses in Ghana. This virus is transmitted by several species of mealybugs when they feed on cacao plants.
Cacao tree with cacao pods attached directly to the tree trunk and branches. Image credit: Gyamera et al., doi: 10.1094/PDIS-10-22-2412-FE.
The cacao swollen shoot virus disease (CSSVD) was first observed in the Eastern Region of Ghana in 1936 by a farmer and its virus nature was confirmed in 1939.
CSSVD is considered the most economically damaging cacao virus disease that could account for 15-50% yield loss if the severe strains are involved in infections.
The cacao swollen shoot virus (CSSV) is classified as a member of the plant-infecting pararetroviruses in the genus Badnavirus which are with non-enveloped bacilliform particles that encapsulate a circular double-stranded DNA-genome.
Previously, the isolates and strains were grouped according to the severity of symptom expression and geographical origin.
Now, it is known that CSSVD is caused by a complex of Badnavirus species based on their molecular structure.
CSSV affects all parts of the cacao plant. The symptoms seen on the leaves include red vein banding of the immature ‘flush’ leaves, chlorotic vein flecking or banding which may occur in angular flecks, chlorotic vein clearing, and various forms of mosaic symptoms.
The virus causes swellings of the stems (nodes, internodes, tips) and roots.
Some strains also cause infected pods to change shape and become rounder, smaller and with smoother surfaces.
“This virus is a real threat to the global supply of chocolate,” said Professor Benito Chen-Charpentier, a researcher at the University of Texas at Arlington.
“Pesticides don’t work well against mealybugs, leaving farmers to try to prevent the spread of the disease by cutting out infected trees and breeding resistant trees. But despite these efforts, Ghana has lost more than 254 million cacao trees in recent years.”
Farmers can combat the mealybugs by giving vaccines to the trees to inoculate them from CSSV. But the vaccines are expensive, especially for low-wage farmers, and vaccinated trees produce a smaller harvest of cacao, compounding the devastation of the virus.
Electron micrograph of purified cacao swollen shoot virus particles. Scale bar – 200 nm. Image credit: Gyamera et al., doi: 10.1094/PDIS-10-22-2412-FE.
Professor Chen-Charpentier and his colleagues developed a new strategy: using mathematical data to determine how far apart farmers can plant vaccinated trees to prevent mealybugs from jumping from one tree to another and spreading the virus.
“Mealybugs have several ways of movement, including moving from canopy to canopy, being carried by ants or blown by the wind,” Professor Chen-Charpentier said.
“What we needed to do was create a model for cacao growers so they could know how far away they could safely plant vaccinated trees from unvaccinated trees in order to prevent the spread of the virus while keeping costs manageable for these small farmers.”
By experimenting with mathematical patterning techniques, the authors created two different types of models that allow farmers to create a protective layer of vaccinated cacao trees around unvaccinated trees.
“While still experimental, these models are exciting because they would help farmers protect their crops while helping them achieve a better harvest,” Professor Chen-Charpentier said.
“This is good for the farmers’ bottom line, as well as our global addiction to chocolate.”
A paper on the findings was published online in the journal PLoS ONE.
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F.B. Agusto et al. 2024. Cacao sustainability: The case of cacao swollen-shoot virus co-infection. PLoS ONE 19 (3): e0294579; doi: 10.1371/journal.pone.0294579