Some of the world’s largest chocolate-producing countries, such as the Ivory Coast, could lose a third of their crop this year, because of the problem.
Researchers are trying to map the DNA of the cacao tree to find genes which could be resistant to the two diseases, reports New Scientist magazine.
One, the cacao swollen shoot virus (CSSV), which can kill the trees, is affecting crops in west Africa.
In Brazil, another major producer of chocolate, crops are being affected by a fungus called witches’ broom.
Made by fermenting and then roasting the seeds of the cacao tree, around 70 per cent of the world’s chocolate comes from west Africa.
In recent years greater number of trees have been planted closer together as farmers try to keep up with increased demand and struggle to afford expensive fertilisers.
In practice, this has meant that other types of trees, which would normally have grown between the cacao trees, have been cut down.
This has encouraged the spread of disease, as has the trend to grow the plants in dry countries far from their native Amazon rainforest, where a lack of water makes them less able to stave off attack.