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Researchers are racing to tackle a virus that threatens to shrink global chocolate supplies and destroy the livelihoods of thousands of West African smallholders.

The cacao swollen shoot virus (CSSV) — which can kill the cacao tree, from whose seeds chocolate is made — threatens to cut this year's spring crop by a third in the Côte d'Ivoire, the world's biggest producer. It originated in native African trees and is spread by common mealy bugs.

Spread of the disease is promoted by African farmers who cannot afford fertiliser and so boost production by planting more of the Amazon-native trees over a larger area — cutting down other species of tree that grow in between and creating a monoculture that promotes the spread of disease.

Previously, the only solution has been to destroy millions of infected trees. But researchers at the West African Cacao Research Institute in Tafo, Ghana, have discovered partially disease-resistant varieties of the tree and are currently trying to breed more resistant strains.

This is a lengthy process so, at the US Department of Agriculture in Miami, Florida, scientists are mapping genes for CSSV resistance. They are hoping that this will eventually result in a "testing kit" for African researchers to ensure that only resistant seedlings are planted.